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LOS  AKliELKS 
OBKARY 


SHORE  PROCESSES 
SHORELINE  DEYELOPMENT 


BY 

DOUGLAS  WILSON   JOHNSON 

Associate  Professor  of  Physiography, 
Columbia  University 


FIRST  EDITION 


NEW  YORK 

JOHN  WILEY  &  SONS,  Inc. 

London:    CHAPMAN   &    HALL,   Limited 

191Q 


Copyright,  1919,  by 
DOUGLAS  W.  JOHKSON 


Copyriyhted  in  Great  Britain 


Stanbope  iprcss 

F.    H.GILSON    COMPANY 
BOSTON,  U.S.A. 


~-o 


r\ 


GB 


-T 


X  PREFACE 

The  present  work  was  born  of  a  need  experienced  by  the  author 
in  connection  with  his  shorehne  studies.  In  the  course  of  a  critical 
cj  examination  of  the  arguments  supposed  by  manj^  to  demonstrate 
a  progressive  subsidence  of  the  Atlantic  coast  of  North  America 
within  historic  time,  it  developed  that  in  respect  to  certain  of  these 
arguments  agreement  between  students  of  the  problem  could  not 
be  reached  because  there  was  not  sufficient  agreement  as  to  what 
features  are  normally  characteristic  of  a  stable  coast,  and  what 
features  are  peculiar  to  coasts  which  are  rising  or  subsiding.  No 
work  existed  which  combined  with  an  extended  analysis  of  the 
forces  operating  along  the  shore,  a  full  and  systematic  discussion 
of  the  cycle  of  shorehne  development  and  such  further  discussion 
of  the  modifying  effects  of  changes  of  level  as  would  enable  one  to 
differentiate  stable,  rising,  and  subsiding  coasts. 

It  seemed  necessary,  therefore,  to  enquire  somewhat  fully  into 
the  fundamental  principles  of  shore  processes  and  shorehne  de- 
velopment; for  it  would  not  be  profitable  to  add  to  the  already 
overburdened  literature  on  changes  of  level  another  essay  which 
should  merely  add  quantitatively  to  the  volume  of  evidence  pre- 
viously discussed  by  many  earlier  writers,  and  again  assert  as  the 
correct  interpretation  of  that  evidence  conclusions  which  some 
geologists  and  geographers  accept  and  others  reject.  Profit  could 
come  from  the  study  only  in  case  the  discussion  of  principles  was 
such  as  to  bring  geologists  and  geographers  into  substantial  agree- 
ment as  to  what  shore  features  are,  and  what  are  not  indicative  of 
changes  of  level.  Once  this  measure  of  agreement  was  reached,  I 
could  not  doubt  that  a  critical  analysis  of  the  arguments  supposed 
to  prove  the  progressive  subsidence  within  historic  time  of  the 
coast  of  southeastern  Canada,  the  Atlantic  coast  of  the  United 
States,  and  certain  other  marginal  areas  of  the  continents,  would 
demonstrate  to  the  impartial  and  critical  student  the  inadequacy 
of  those  arguments.  I  therefore  set  myself  the  task  of  bringing 
together  the  results  of  shoreline  studies  published  in  different 
languages,  of  analyzing  and  criticising  the  conclusions  reached 


iv  PREFACE 

where  this  might  appear  to  be  profitable,  and  of  presenting  a  digest 
of  those  fundamental  principles  which  should  prove  to  be  best 
established  by  the  independent  work  of  different  students  and 
best  supported  by  my  own  field  observations.  At  the  seme  time 
I  purposed  to  develop  somewhat  fully  certain  important  aspects  of 
the  physiography  of  shorehnes  wliich  have  hitherto  received  little 
consideration. 

The  magnitude  of  the  task  proved  to  be  greater  than  antici- 
pated, partly  because  of  the  wide  divergence  of  expert  opinion 
regarding  the  manner  in  which  shore  processes  operate,  and  partly 
because  of  the  great  volume  and  scattered  distribution  of  the 
writings  dealing  with  the  subject.  It  was,  indeed,  the  desire  to 
relieve  others  who  might  have  occasion  to  study  shore  processes 
and  shoreline  forms,  of  the  burden  of  dupHcating  the  work  involved 
in  my  undertaldng  which  first  suggested  to  me  the  desirability  of 
placing  on  record,  in  compact  form  for  their  use,  the  results  of  my 
enquiry,  even  where  these  results  did  not  relate  to  the  original 
problem  of  coastal  subsidence.  The  present  volume  is  the  con- 
crete product  of  this  desire  to  render  a  service  to  my  fellow 
students. 

The  engineer  will  find  in  the  chapters  on  waves  and  currents  a 
summary  of  the  widely  conflicting  opinions  and  observations  re- 
lating to  those  most  puzzling  forces  with  which  he  has  to  deal. 
In  the  later  chapters  he  will  also  find,  I  hope,  not  a  few  discussions 
of  shore  forms  and  of  the  method  of  their  development  which  will 
prove  useful  to  him  in  his  work  on  marine  engineering  structures. 
The  dynamic  geologist  will  find  in  the  first  chapters  an  extended 
account  of  two  of  the  forces  of  nature  with  which  he  is  much  con- 
cerned, and  in  the  remaining  chapters  abundant  illustrations  of 
the  manner  in  which  those  forces  operate  near  the  margins  of  the 
lands.  The  geographer  will  be  mainly  concerned  wdth  the  last 
seven  chapters  where  the  forms  of  the  shorehne  receive  a  syste- 
matic treatment  which,  if  not  adequate,  is  at  least  somewhat  more 
detailed  and  complete  than  any  hitherto  attempted.  Throughout 
the  volume  the  reader  will  note  that  repeatedly  conclusions 
reached  and  principles  established  are  briefly  applied  to  the  prob- 
lem of  changes  of  level;  and  he  will  understand  that  this  is  the 
thread,  appearing  now  and  then,  which  is  to  connect  parts  of  the 
present  study  with  a  later  volume  devoted  exclusively  to  the  much 
mooted  question  of  coastal  subsidence. 


PREFACE  V 

As  a  rule  an  advance  summary  precedes,  and  a  brief  resume  con- 
cludes the  text  of  each  chapter.  This  will  enable  engineer,  geolo- 
gist and  geographer  to  determine  in  some  measure  the  extent  to 
which  matters  pertinent  to  their  respective  fields  are  discussed. 
A  bibliography,  arranged  alphabetically  according  to  authors  and 
placed  at  the  end  of  the  volume,  supplements  the  Ust  of  references 
given  at  the  close  of  each  chapter.  Finally,  an  index  of  authors 
and  an  index  of  subjects  are  provided  in  the  form  which  it  is  hoped 
will  prove  most  serviceable  to  the  reader. 

In  any  attempt  to  give  proper  credit  for  the  aid  rendered  by 
others  during  the  preparation  of  this  volume  the  writer  is  much 
embarrassed.     The  work  of  preparation  has  extended  over  several 
years,  during  which  time  a  number  of  students,  colleagues  and 
friends  have  been  most  generous  in  rendering  valuable  assistance. 
It  would  be  impossible  to  make  specific  acknowledgments  to  all  of 
them,  so  great  is  the  measure  of  my  indebtedness.     Special  thanks 
are  due  to  my  cousin.  Miss  Laura  Dale  Johnson,  for  assuming 
the  labor  of  reading  the  proofs  and  seeing  the  book  through  the 
press  during  my  absence;  to  Miss  Florrie  Holzwasser  of  the  De- 
partment of  Geology  of  Barnard  College,  and  others  among  my 
graduate  students,  for  assistance  in  reviewing  and  abstracting  the 
literature  relating  to  the  subject  in  hand;  and  to  Dr.  A.  K.  Lobeck 
for  preparing  the  five  block  diagrams  showing  successive  stages 
in  the  development  of  a  shoreline  of  submergence.     Acknowledg- 
ments should  be  made  to  ''The  Geographical  Review,"  "Science," 
the  "Bulletin  of  the  Geological  Society  of  America,"  and  the 
"Journal  of  Geology"  for  the  use  of  certain  material  originally 
published  in  their  pages.     Many  of  the  observations  recorded  in 
this  volume  were  made  in  the  course  of  a  special  Shaler  Memorial 
Investigation  of  the  problem  of  coastal  subsidence  undertaken 
with  the  support  of  the  Shaler  Memorial  Fund  of  Harvard  Uni- 
versity; and  observations  on  the  New  Jersey  coast  were  obtained 
in  connection  with  a  study  in  progress  for  the  Geological  Survey 
of  New  Jersey  under  the  direction  of  Dr.  H.  B.  Klimmel.     It  is  a 
pleasure  to  express  special  obligations  to  Professor  W.  M.  Davis 
for  helpful  criticism  of  the  manuscript,  and  to  acknowledge  the 
debt  wliich  all  physiographers  owe  to  his  studies  of  shoreline 
topography,  which  were  the  first  to  demonstrate  the  value  of 
applying  the  idea  of  the  cycle  to  the  history  of  shore  forms.     To 
Professor  Joseph  Barrell,  whose  studies  have  to  some  extent  paral- 


vi  PREFACE 

leled  certain  of  my  own,  I  am  indebted  for  many  valuable  sugges- 
tions and  for  his  generous  courtesy  in  giving  to  the  manuscript  a 
careful  and  critical  reading,  from  the  results  of  which  I  have  greatly 
profited. 

In  conclusion  it  is  but  fair  to  acknowledge  the  author's  keen 
appreciation  of  certain  defects  which  the  reader  may  discover  in 
his  perusal  of  the  text.  The  volume  goes  to  press  under  circum- 
stances which  absolutely  prevent  that  careful  attention  to  details 
which  every  work  of  this  kind  should  receive.  On  entering  the 
service  of  his  country  the  writer  was  forced  to  choose  between 
publishing  his  studies  without  the  final  supervision  which  he  had 
hoped  to  give  the  proofs,  and  postponing  pubHcation  indefinitely. 
In  view  of  the  uncertainties  attending  service  in  the  zones  of  mili- 
tary operations,  it  has  seemed  wiser  to  allow  the  work  to  go  to 
press,  in  the  hope  that  the  indulgent  reader  will  not  find  the  value 
of  the  volume  materially  affected  by  such  errors  in  execution  as 
the  presence  of  the  author  alone  could  have  prevented. 

DOUGLAS  WILSON  JOHNSON. 
On  Board  Troop  Ship, 
April  3,  1918. 


CONTENTS 

Page 

List  of  Plates ix 

List  of  Illustrations xiii 

CHAPTER  I 

Water  Waves 1 

Advance  summary,  1;  Scope  of  subject,  2;  Literature,  4;  Waves 
of  oscillation,  7;  Origin,  7;  Wave  motion,  8;  Wave  form,  12;  Wave 
height,  21;  Wave  length,  27;  Wave  velocity,  29;  Waves  of  transla- 
tion, 33;  Earthquake  and  explosion  waves,  38;  Tidal  waves,  41; 
Standing  waves;  seiches,  42;  Boundary  waves,  44;  Resume,  45; 
References,  46. 

CHAPTER   n 

The  Work  of  Waves 55 

Advance  summary,  55;  Wave  energy,  55;  Nature  of  wave  attack, 
57;  Wave  dynamometer,  62;  Measurements  of  wave  energy,  63; 
Damage  by  storm  waves,  65;  Conditions  affecting  wave  energy, 
72;  Wave  refraction,  74;  Depth  of  wave  action,  76;  Resume,  83; 
References,  83. 

CHAPTER   III 

Current  Action 87 

Advance  summary,  87;  Types  of  currents,  88;  Wave  currents,  90; 
Tidal  currents,  106;  Seiche  currents,  122;  Wind  currents,  123; 
Planetary  currents,  128;  Pressure  currents,  1.30;  Convection  cur- 
rents, 131;  Salinity  currents,  131;  River  currents,  136;  Reaction 
currents,  138;  Eddy  currents,  139;  Deflection  of  currents,  141; 
Resume,  148;  References,  149. 


CHAPTER   IV 

Terminology  and  Classification  of  Shores 159 

Advance  summary,  159;  Terminology  of  shores,  159;  Plains, 
planes,  and  peneplanes,  164;  Classification  of  shorelines,  169;  I. 
Shorelines  of  submergence,  173;  II.  Shorelines  of  emergence,  186; 
III.  Neutral  shorelines,  187;  IV.  Compound  shorelines,  190; 
Stages  of  shoreUne  development,  192;  Resume,  192;  References, 
192. 


■a 


/ 


Vlll  CONTENTS 

CHAPTER  V 

Page 

Development  of  the  Shore  Profile 199 

Shorelines  of  submergence,  199;  Advance  summary,  199;  Initial 
stage,  201;  Young  stage,  203;  Mature  stage,  210;  Old  stage,  224; 
Validity  of  the  theory  of  a  marine  cycle,  228;  Correlation  of  the 
marine  anil  fluvial  cycles,  242;  Independence  of  marine  and  fluvial 
cycles,  245;  Comparative  rapidity  of  marine  and  fluvial  planation, 
249;  Probability  of  marine  planation,  253;  Interruptions  and  acci- 
dents during  the  marine  cycle,  257;  Shorelines  of  emergence,  258; 
Initial  stage,  258;  Young  stage,  259;  Mature  and  old  stages,  262; 
Neutral  shorelines,  262;  Compound  shorelines,  265;  Resume,  267; 
References,  268. 

CHAPTER  VI 

Development  of  the  Shoreline 272 

A.  Shorelines  of  submergence,  272;  Advance  summary,  272; 
Initial  stage,  272;  Young  stage,  275;  Stages  of  development  of 
shore  details,  328;  Relative  importance  of  different  marine  forces  in 
the  formation  of  bars,  forelands,  etc.,  333;  Mature  stage,  339; 
Old  stage,  344;  Resume,  345;  References,  345. 

CHAPTER  VII 

Development  of  the  Shoreline  (Conlitiued) 348 

B.  Shorelines  of  emergence,  348;  Advance  summary,  348;  Initial 
stage,  348;  Young  stage,  350;  Effect  of  progressive  subsidence  on 
lagoon  history,  383;  Effect  of  progressive  elevation  on  lagoon  his- 
tory, 386;  Offshore  bars  not  an  evidence  of  subsidence,  386;  Mature 
stage,  389;  Old  stage,  390;  Resume,  392;  References,  392. 

CHAPTER  Vm 

Development  of  the  Shoreline  (Continued) 395 

C.  Neutral  and  compound  shorelines,  395;  Neutral  shorelines, 
395;  Compound  shoreUnes,  400;  Contraposed  shorehnes,  401; 
References,  403. 

CHAPTER  IX 

Shore  Ridges  and  Their  Significance 404 

Advance  summary,  404;  Origin  of  beach  ridges,  404;  Rate  of 
beach  ridge  formation,  414;  Beach  ridges  as  records  of  changes  of 
level,  439;  Resume,  453;  References,  454. 

CHAPTER   X 

Minor  Shore  Forms 458 

Advance  summary,  458;  Beach  cusps,  458;  Low  and  ball,  486; 
Ripple  marks,  489;  Rill  marks,  512;  Swash  marks,  513;  Backwash 
marks,  517;  Sand  domes,  518;  Shore  dunes,  519;  Resume,  525; 
References,  525. 


LIST   OF   PLATES 

Page 
I.   Storm  waves  breaking  against  seawall  at  Hastings,  England.       14 
II.   Combing  wave,  showing  water  completing  orbital  move- 
ment although  insufficient  in  quantity  to  fill  the  wave 


form. 


17 


III.  Waves  breaking  against  seawall  at  Scarborough,  England. .        19 

IV.  Stormwavestrikingfaceof  seawall  at  Scarborough,  England       59 
V.   Water  forced  vertically  upward  by  wave  breaking  against 

seawall  at  Scarborough,  England 61 

VI.    Marine  Cliff  at  Highland  Light,  Cape  Cod,  rapidly  retreat- 
ing under  wave  attack 64 

VII.    Shakespeare's  Cliff  near  Dover,  England 79 

VIII.   Wire  fence  undermined  by  wave  attack  and  left  hanging  in 

mid-air '  "^ 

IX.   Cobblestones  cast  into  a  high  ridge  well  above  sealevel  by 

storm  waves '  ° 

X.   Cobblestones  cast  upon  the  beach  from  deep  water  through 
the  combined  effect  of  wave  action  and  the  buoyancy  of 

attached  seaweeds 92 

XI.   Surf  breaking  on  the  shore  of  Cape  Canaveral,  Florida 95 

XII.   Winthrop  Great  Head,  a  wave-chffed  drumlin  near  Boston, 

Massachusetts 98 

XIII.  Lowestoft  Ness  on  the  east  coast  of  England .  102 

XIV.  Giant  sand  ripples  produced  by  strong  ebb  tide  current  in 

the  Avon  River  estuary  near  Windsor,  Nova  Scotia 110 

XV.   Salt  marsh  near  Green  Harbor,  Massachusetts 112 

XVI.   Hornviken,  a  small  fjord  near  North  Cape,  Norway,  showing 

typical  oversteepened  walls  of  a  glacial  trough 175 

XVII.   Idde  Fjord  near  Fredriksten,  Norway,  showing  rectangular 

pattern  characteristic  of  many  fjord  coasts 178 

XVIII.   The  Naero  Fjord,  Norway,  a  partially  submerged  glacial 

trough •     18^ 

XIX.   Lake  Loen,  Norway,  occupying  a  glacial  trough  and  practi- 
cally continuous  with  the  upper  part  of  Nord  Fjord ....      183 
XX.   Stromstad  Harbor,  Sweden,  showing  characteristics  of  a 

fiard  coast ^°^ 

XXI.   Northwestern  coast  of  France  near  Fecamp,  showing  youth- 
ful cUff  profiles  on  a  mature  shoreline  of  submergence.  .  .     200 
XXII.    Same  view  as  Plate  XXI,  but  at  low  tide,  showing  marine 

cliff,  wave-cut  bench,  and  narrow  beach  at  base  of  cliff .  .     202 
XXIII.    Marine  cliff  cut  in  glacial  drift  near  Plymouth  on  the  Mas- 
sachusetts coast ■ ^^'^ 


X  LIST  OF  PLATES 

Page 
XXIV.   Marine  cliff  cut  in  sand  and  clays  of  the  coastal  plain  near 

Beaufort,  North  Carolina 207 

XXV.  Marine  cliff  cut  in  sand  dunes  on  the  shore  of  Cape  Cana- 
veral, Florida 1 209 

XXVI.  View  near  Rhuvaal,  Islay,  off  the  west  coast  of  Scotland, 
showing  former  marine  cliff  and  wave-cut  bench,  recently 

elevated  above  sealevel 227 

XXVII.    Elevated  marine  cliff  near  Oban,  Scotland 233 

XXVIII.    Marine  cliff  and  wave-cut  rock  bench  on  the  Pacific  coast 

south  of  Cape  Flattery,  Washington 236 

XXIX.   Marine  peneplane  and  monadnock  near  Madura,  east  coast 

of  India 239 

XXX.   Base  of  monadnock  in  Plate  XXIX,  showing  effects  of 

marine  erosion 241 

XXXI.    Monadnock  on  marine  peneplane  of  the  east  coast  of  India .     251 
XXXII.    Imposing  marine  cliff  at  North  Cape,  Norway 252 

XXXIII.  Rocky  headland  and  drowned  valley  of  a  young  shoreline  of 

submergence,  near  Clifton,  Massachusetts 255 

XXXIV.  Stack  or  chimney  in  front  of  a  young  cliff  on  the  coast  of 

France 277 

XXXV.   The  Dogstone,  near  Oban,  Scotland,  a  stack  in  front  of  a 

marine  clifT,  now  elevated  25  feet  above  sealevel 279 

XXXVI.   Fingal's  Cave  on  the  Island  of  Staffa,  western  Scotland.  .  .  .     280 
XXXVII.   Ancient  sea  cave  on  an  elevated  shoreline  of  western  Scot- 
land, transformed  into  a  stable 282 

XXXVIII.    Wave-cut  arch  on  the  northwest  coast  of  France 284 

XXXIX.   Third  Cliff  near  Scituate,   Massachusetts,  showing  small 

landslide  due  to  wave  erosion  of  cliff  base 286 

XL.   Bay-head  or  pocket  beach  near  Rye,  New  Hampshire 288 

XLI.    Deltas  of  cobblestone  formed  by  overwash  of  storm  waves 

near  Marblehead,  Massachusetts 305 

XLII.   Tombolo  connecting  the  former  island  of  Marblehead  with  '' 

the  Massachusetts  mainland 314 

XLIII.  Hanging  valleys  on  the  chalk  coast  of  southeastern  England, 
where  waves  cut  back  the  shore  faster  than  the  streams 

can  deepen  their  valleys 342 

XLIV.  Former  offshore  bar  near  Wrightsville,  North  Carolina, 
rising  above  the  marsh  surface  back  of  the  present  off- 
shore bar 353 

XLV.  Surface  of  a  salt  marsh  near  Boston,  Massachusetts,  which 
overlies  a  peat  deposit  20  feet  deep  composed^  of  remains 

of  high-tide  vegetation 384 

XLVI.    Contraposed  shoreline  south  of  Rye  Beach,  New  Hampshire.     402 
XLVII.   Ancient  beach  ridge   (center  of  view)   connecting  former 

island  in  distance  with  one  in  foreground 406 

XLVIII.  Ancient  series  of  dune  ridges  on  Cape  Canaveral,  truncated 
at  right  angles  by  a  later  series  in  the  foreground,  on  one 
member  of  which  the  man  is  sitting 415 


LIST  OF  PLATES  XI 

Page 
XLIX.   Shingle  beach  ridges  of  the  Dungeness  cuspate  forelaiid, 

England 421 

L.   Shingle  beach  ridges  on  the  island  of  Riigen,  Germany ....     425 
LI.   Forested  dune  ridges  on  the  Darss  cuspate  foreland,  Ger- 
many, showing  marked  inequality  in  altitude  of  crestlines     427 
LII.   Road-cut  through  an  old  dune  ridge  at  the  back  of  the 

present  shore  at  Daytona,  Florida 432 

LIII.    Successive  curving  beach  ridges  and  swales  forming  lines  of 

growth  on  Cape  Canaveral 441 

LIV.    Level  surface  of  Skanor  peninsula,  southwestern  Sweden.     448 
LV.    Beach  cusps  of  gravel  on  the  shore  of  Carmel  Bay,  California     452 
LVI.   Giant  sand  cusps  on  Melbourne  Beach,  Florida,  truncated 

2    by  wave  erosion 464 

LVII.   Cusps  on  Nantasket  Beach,  Massachusetts 468 

LVIII.    Beach  cusps  on  the  west  coast  of  Porto  Rico,  near  Melones 

Point 472 

LIX.   Sandstone  slab  showing  fossil  oscillation  ripples 490 

LX.    Current  ripples  formed  by  an  ebbing  tide 492 

LXI.    Current  ripples  on  the  shore  of  Nantasket  Beach,  Massa- 
chusetts       493 

LXII.   Giant    current    ripples    near    Annisquam,    Massachusetts, 
showing  irregular  pattern  due  to  interference  of  wave  and 

tidal  currents 499 

LXIII.   Current  ripples  near  Windsor,  Nova  Scotia 503 

LXIV.   Current  ripples  modified  by  later  oblique  wave  or  current 

action 506 

LXV.   Plaster  cast  of  interference  ripple  mark  formed  by  two 

systems  of  waves  crossing  nearly  at  right  angles 507 

LXVI.   Rill  marks  on  a  sandy  beach 511 

LXVII.   Plaster  cast  of  swash  marks  left  by  four  successive  waves 

on  the  sandy  shore  of  Lake  Erie 514 

LXVIII.   Cast  of  backwash  marks 515 

LXIX.   Plaster  cast  of  backwash  marks 516 

LXX.   Shore  dunes  of  the  Holland  coast  near  Katwijk 520 

LXXI.   Shore  dunes  near  Scheveningen,  Holland 521 

LXXII.  Dune  of  barchane  form  overwhelming  trees  on  the  Province- 
lands  of  Cape  Cod 522 

LXXIII.   Shore  dunes  near  Cape  Henry,  Virginia,  migrating  miand 

over  the  forest 524 


LIST   OF   ILLUSTRATIONS 

Frontispiece.     Wave-cut  bench  and  marine   cliff    bordering  Desecheo 
Island,  Porto  Rico,  recently  elevated  above  sealevel. 
Fig.  Page 

1.  Diagram  showing  the  motion  of  water  particles  in  an  oscillatory 

wave 9 

2.  Diagram  showing  the  elliptical  orbits  of  water  particles  in  shallow- 

water  waves,  and  the  decrease  in  size  of  orbits  with  increasing 
depths 10 

3.  Diagram  showing  theoretical  form  of  a  cycloidal  wave,  and  the 

rapid  decrease  in  size  of  the  orbits  (through  which  the  water  par- 
ticles move)  with  increasing  depth 12 

4.  Theoretical  profiles  of  three  trochoidal  waves  having  different  sized 

orbits    (solid-line   profile  and  broken-line  profile),   or  different 
spacing  of  orbits   (broken-line  profile  and  dotted-line  profile).       13 

5.  Diagram  showing  how  two  regular  series  of  waves  of  different 

heights  and  lengths  combine  to  form  an  irregular  series 26 

6.  Diagram  showing  movement  of  water  particles  in  a  wave  of  trans- 

lation        34 

7.  Diagram  showing  how  oscillatory  waves  breaking  on  a  subaqueous 

terrace  produce  waves  of  translation 37 

8.  Diagram  to  illustrate  the  movement  of  water  particles  in  standing 

waves,  such  as  the  seiche 43 

9.  Boundary  wave  formed  by  local  air  current  over  liquids  of  different 

densities ,        44 

10.  Diagram  showing  movement  of  water  particles  in  overlying  fresh 

water  and  underlying  salt  water  during  the  passage  of  bound- 
ary waves  from  left  to  right 44 

11.  Stevenson's  wave  dynamometer 62 

12.  Diagram  to  illustrate  the  process  of  wave  refraction,  whereby  wave 

attack  is  concentrated  on  headlands 75 

13.  Section  of  beach  slope  showing  by  dotted  lines  the  so-cailed  zig-zag 

path  of  debris  particles  during  beach  drifting  and  by  solid  lines 

the  parabolic  paths  actually  followed 94 

14.  Parabolic  paths  of  large  and  small  particles  of  debris  subject  to 

beach  drifting ,       96 

15.  Parabolic  paths  followed  by  debris  particles  impelled  by  tne  oom- 

bined  action  of  on-shore  swells  and  oblique  wind  waves 97 

16.  Diagram  to  illustrate  relation  of  beach  drifting  to  wind  directions  100 

17.  Parabolic  paths  of  debris  particles  subject  to  beach  drifting 101 

18.  Elliptical  orbit  of  water  particle  during  passage  of  the  tide  wave 

over  a  sloping  seabottom 106 

19.  Course  followed  by  a  nearly  submerged  float  under  the  influence 

of  tidal  currents  in  New  York  harbor ,    .  „ . . . .     116 

xiii 


XIV  LIST  OF  ILLUSTRATIONS 

Fig.  Page 

20.  Theoretical  course  calculated  by  Parsons  for  the  float  whose  actual 

course  is  shown  in  Figure  19 118 

21.  Eddy  currents  in  the  Gulf  of  Honduras  and  Mosquito  Gulf 140 

12.   Elements  of  the  shore  zones  during  an  early  stage  of  development.  160 

23.  Elements  of  the  shore  zones  in  an  advanced  stage  of  development.  162 

24.  Embayed  coastal  plain  of  Chesapeake  Bay  region,  showing  example 

of  ria  shoreline 174 

25.  A  typical  section  of  the  fjord  coast  of  Norway,  showing  angular 

pattern  attributed  to  fault-control 177 

26.  Mississippi  Delta 187 

27.  Delta  of  the  Tiber 188 

28.  Niger  Delta 189 

29.  Initial  stage  of  a  fault  shoreline 190 

30.  Coast  of  North  Carolina,  showing  one  type  of  compound  shoreline.  191 

31.  Compound  shoreline  due  to  faulting  and  partial  submergence  of  up- 

throw block 191 

32.  Stages  in  the  development  of  the  shore  profile  of  a  shoreline  of  sub- 

mergence    201 

33.  Successive  profiles  of  equilibrium  on  a  retrograding  shore 211 

34.  Profiles  of  equilibrium  off  the  Madagascar  coast  as  plotted  from 

charts  by  Barrell 212 

35.  Stages  in  the  development  of  the  shore  profile  of  a  shoreline  of  sub- 


mergence . 


213 

36.  Variations  in  the  beach  profile  of  equilibrium  due  to  variations  in 

the  different  shore  forces 218 

37.  Mature  and  old  shore  profiles  of  a  shoreline  of  submergence 224 

38.  San  Clemente  Island  off  the  coast  of  southern  California,  showing 

series  of  uplifted  wave-cut  terraces 229 

39.  Cross-section  of  the  western  coast  and  continental  shelf  of  Norway.  232 

40.  Stages  in  the  reduction  of  a  land  mass 256 

41.  Overlapping  of  marine  deposits  upon  the  abrasion  platform  of  a 

slowly  subsiding  land  mass 258 

42.  Elements  of  the  profile  of  a  shoreline  of  emergence 258 

43.  Stages  in  the  development  of  the  shore  profile  of  a  fault  coast ....  264 

44.  Profile  of  a  shoreline  of  emergence 266 

45.  Shoreline  of  submergence,  initial  stage 273 

46.  ShoreUne  of  submergence  in  Chesapeake  Bay  region 274 

47.  Early  youth  of  a  shoreline  of  submergence,  showing  crenulate  shore- 

line    275 

48.  Crenulate  shoreline  of  the  southwest  coast  of  Ireland 276 

49.  Young  shoreline  of  submergence  near  Idzuhara,  Japan,  showing 

crenulate  stage 278 

50.  Young  shoreline  of  submergence,  showing  types  of  beaches,  bars, 

spits,  and  forelands 283 

51.  Initial  unorganized  condition  of  currents  along  a  young  shoreline 

ot  submergence  compared  with  organized  condition  which  ob- 
tains when  the  stages  of  submaturity  or  maturity  are  reached. .  .  285 

52.  Sand  spits  on  the  shore  of  Port  Orchard,  Washington 287 


LIST  OF  ILLUSTRATIONS  XV 

Fig.  Page 

53.  Simple  spit  and  compound  recurved  spit  at   entrance  to   Port 

Moller,  Alaska 289 

54.  Successive  stages  in  the  development  of  one  type  of  compound  re- 

curved spit 291 

55.  Compound  recurved  spit  enclosing  Toronto  Harbor 293 

56.  Development  of  the  Cape  Cod  shoreline 294 

57.  Development  of  Sandy  Hook  spit 296 

58.  Lagoons  and  ridges  of  the  Presque  Isle  compound  recurved  spit.. .  299 

59.  Spits  built  by  converging  currents 300 

60.  Spits  converging  to  form  a  bay  bar  on  the  Alaskan  coast 300 

61.  Bay-mouth  bars  on  the  Marthas  Vineyard  coast 301 

62.  Bay-mouth  bar  on  the  Marthas  Vineyard  coast 302 

63.  Bay-head  bar  near  Duluth 303 

64.  Mid-bay  bar  in  Hempstead  Harbor,  Long  Island 304 

65.  Winged  headland  near  Sag  Harbor,  Long  Island 306 

66.  Offsets,  overlaps,  and  stream  deflection 308 

67.  Looped  bar  on  shore  of  Shapka  Island,  Alaska 310 

68.  Renard  Island  near  Seward,  Alaska,  showing  embankment  growing 

from  island  toward  mainland 311 

69.  Inner  Iliasik  Island,  Alaska,  showing  embankment  which  may  be 

upbuilding  toward  the  surface  simultaneously  along  its  entire 

length 311 

70.  Single  tombolo  connecting  former  island  of  Marblehead  with  the 

mainland 312 

71.  Former  island  of  Big  Nahant  tied  to  Little  Nahant,  and  the  latter 

to  the  mainland  by  single  tombolo 313 

72.  Duxbury  and  Saquish  Neck  tombolos  uniting  former  islands  with 

the  mainland  of  Massachusetts  near  Plymouth 316 

73.  Monte  Argentario,  Italy,  tied  to  the  mainland  by  a  double  tombolo  317 

74.  Morro  del  Puerto  Santo,  Venezuela,  a  Y-tombolo 318 

75.  Former  islands,  many  of  which  were  wholly  or  completely  eroded  by 

wave  action,  and  the  resulting  debris  used  by  the  waves  to  build 

a  complex  tombolo  tying  the  remaining  islands  to  the  mainland.  319 

76.  Nantasket  Beach,  Massachusetts,  the  complex  tombolo  formed  by 

wave  erosion  of  the  islands  shown  in  Fig.  75,  with  deposition  of 
the  debris  to  give  connecting  beaches  uniting  the  remaining 

islands  with  each  other  and  with  the  mainland  at  the  south.  320 

77.  A  strongly  recurved  spit  on  the  Washington  coast,  about  to  become 

a  cuspate  bar 321 

78.  Cuspate  bars  on  the  Narragansett  Bay  shore 321 

79.  Cuspate  bar,  showing  enclosed  marsh  near  Providence,   Rhode 

Island 321 

80.  Cuspate  bar  originally  built  as  a  tombolo  tying  to  the  mainland  an 

island  since  removed  by  wave  erosion 321 

81.  Cuspate  bars  on  the  shores  of  Port  Discovery,  Washington 323 

82.  Cuspate  foreland  near  Port  Townsend,  Washington 324 

83.  Types  of  cuspate  foreland  bars 324 

84.  The  former  Cape  Canaveral,  now  known  as  False  Cape 326 


XVI  LIST  OF  ILLUSTRATIONS 

Fig.  Page 

85.  Marsh  bars  on  the  Delaware  Bay  shore 327 

Comparison  of  text  figures  to  facihtate  correlations  of  successive 

stages  in  the  development  of  a  shoreline  of  submergence 331 

86.  Lake  Balaton  (Flatten  Lake),  showing  position  of  cuspate  bar  .  .  .  337 

87.  ShoreUne  of  submergence,  submature  stage 340 

88.  Shoreline  of  submergence,  mature  stage 341 

89.  Valleuses  on  the  northwest  coast  of  France 343 

90.  Theoretical  profile  through  offshore  bar 357 

91.  Theoretical  profile  through  offshore  bar 357 

92.  Theoretical  profile  through  offshore  bar 357 

93.  Theoretical  profile  through  offshore  bar 358 

94.  Theoretical  profile  through  offshore  bar 358 

95.  Theoretical  profile  through  offshore  bar 359 

96.  Theoretical  profile  through  offshore  bar 359 

97.  Profile  through  offshore  bar '. .  .  361 

98.  Profile  through  off.shore  bar 361 

99.  Profile  through  offshore  bar 361 

100.  Profile  through  offshore  bar 361 

101.  Profile  through  offshore  bar 361 

102.  Profile  through  offshore  bar 363 

103.  Profile  through  offshore  bar 363 

104.  Profile  through  offshore  bar 363 

105.  Profile  through  offshore  bar 363 

106.  Profile  through  offshore  bar 363 

107.  Profile  through  offshore  bar 363 

108.  Profile  through  offshore  bar 364 

109.  Profile  through  offshore  bar 364 

110.  Profile  through  offshore  bar 364 

111.  Profile  through  offshore  bar 364 

112.  Profile  through  offshore  bar 364 

113.  Profile  through  offshore  bar 364 

114.  Profile  through  offshore  bar 364 

115.  Offshore  bar  and  lagoon  of  the  Long  Island  coast,  showing  distribu- 

tion of  tidal  inlets 371 

16.  Offshore  bar  and  lagoon  of  the  Xew  Jersey  coast 371 

17.  Tidal  delta  at  Ocracoke  Inlet,  North  Carolina  coast 375 

18.  Stages  in  the  development  and  retrogression  of  an  offshore  bar.  376 
.19.    Stages  in  the  normal  history  of  an  offshore  bar,  due  account  being 

taken  of  the  effect  of  migrating  inlets 378 

120 .  Diagram  showing  how  wave  erosion  of  a  lobate  delta  may  transform 

it  into  an  arcuate  delta  (broken  line) 396 

121.  Fault  shoreline  bordering  a  scarp  which  dies  out  toward  the  right  397 

122.  Similar  to  Fig.  121,  except  that  the  fault  traversed  a  little  dissected 

plain  of  faint  relief 398 

123.  Successive  stages  in  the  retrogre.ssion  of  a  fault  shoreline  bordering 

rocks  of  varying  resistance 399 

124.  Compound  shoreline,  combining  essential  features  of  a  shoreline  of 

submergence  and  a  fault  shoreline 401 


LIST  OF   ILLUSTRATIONS  XVll 

FiQ.  Page 

125.  Stages  in  the  formation  of  a  contraposed  shoreline 403 

126.  Cuspate  delta  of  the  Tagliainento  River,  Italy,  showing  parallel 

beach  ridges 410 

127.  Successive  stages  in  the  development  of  Rockaway  sand  spit,  Long 

Island 417 

128.  Diagram  of  cliffed  headland  and  associated  beach  ridge  plain,  show- 

ing that  one  series  of  ridges  truncating  another  does  not  neces- 
sarily imply  a  longer  lapse  of  time  than  an  equal  number  of 

parallel  ridges 418 

129.  Ridges  of  the  Cape  Canaveral  cuspate  foreland 420 

130.  The  Dungeness  cuspate  foreland,  showing  shingle  beach  ridges  and 

swales 423 

131.  Dune  ridges  of  the  Darss  cuspate  foreland,  Germany 429 

132.  Dune  ridges  of  the  Swinemiinde  tombolo 434 

133.  Beach  ridges  indicating  coastal  emergence 446 

134.  Beach  ridges  indicating  coastal  submergence 447 

135.  Hypothetical  case  in  which  beach  ridges  on  a  rising  coast  may  give 

a  false  indication  of  stability 449 

136.  Hypothetical  case  in  which  beach  ridges  on  a  sinking  coast  give  a 

false  intUcation  of  stability 450 

137.  Types  of  beach  ridges  formed  on  a  stable  coast 451 

138.  Beach  ridges  of  equal  height  separated  by  swales  of  different  depths 

due  to  variations  in  spacing  of  ridges 453 

139.  Diagram  illustrating  Branner's  theory  of  beach  cusp  formation.  .  .  461 

140.  Diagram  illustrating  Branner's  theory  of  the  formation  of  unequally 

spaced  beach  cusps 461 

141.  Variations  in  the  form  of  beach  cusps 465 

142.  Partially  eroded  older  cusps  and  respaced  later  series 466 

143.  Normal  and  inverted  beach  cusps 473 

144.  Artificial  beach  cusps 476 

145.  Beach  cusps  (after  Jefferson)  showing  compound  cusps  at  right. .  .  484 

146.  Oscillation  ripples 491 

147.  Current  ripples 494 

148.  Vortices  involved  in  the  formation  of  current  ripple  mark 497 

149.  Sand  dome 518 


SHORE  PROCESSES  AND 
SHORELINE  DEVELOPMENT 


CHAPTER  I 
WATER   WAVES 

Advance  Summary.  —  No  adequate  appreciation  of  the  many 
problems  presented  by  the  shoreUne  can  be  gained  until  one 
is  familiar  with  the  work  of  waves  and  currents.  The  relative 
importance  of  these  two  forces  in  shaping  the  shore  is  a  much 
disputed  point;  and  the  difficulties  involved  can  best  be  set 
forth,  and  an  attempt  at  their  solution  can  best  be  made,  if  we 
review  the  essential  characters  of  waves  and  currents  with  some 
care,  and  critically  examine  the  manner  in  which  each  operates. 
We  will  first  turn  our  attention  to  the  phenomena  of  water  waves 
of  different  types;  then  we  will  be  in  a  position  to  discuss  the 
work  accomplished  by  such  waves;  after  which  currents  and 
their  work  will  be  considered. 

In  this  first  chapter,  after  a  note  on  the  general  scope  of  the 
present  treatment  of  waves,  there  is  presented  to  the  reader  a 
brief  survey  of  the  literature  on  the  subject,  which  may  be 
useful  in  showing  the  growth  of  our  knowledge  of  waves  since 
the  time  of  Leonardo  da  Vinci.  Attention  is  then  directed  to 
the  two  types  of  waves  which  are  most  effective  in  shore  proc- 
esses: the  wave  of  oscillation  and  the  wave  of  translation.  In 
each  case  the  origin  and  nature  of  the  water  movement  are  ex- 
plained and  the  elements  of  wave  form  are  described.  The 
depths  at  which  waves  break  on  approaching  a  coast  determine 
the  position  of  certain  shore  forms,  and  therefore  receive  con- 
sideration. The  factors  affecting  the  height  of  waves  are  of 
vital  interest  to  the  engineer  employed  on  harbor  works  or 
coast  defenses,  and  to  the  student  of  shore  forms  produced 
under  varied  conditions  of  wave  attack;    hence  these  factors 

1 


2  WATER  WAVES 

are  discussed  with  some  fullness.  A  wave's  capacity  for  de- 
struction depends  upon  both  its  height  and  its  length,  and  the 
velocities  of  certain  waves  vary  with  wave  length  according  to 
definite  laws.  The  lengths  of  waves  and  their  velocities  are 
therefore  matters  of  importance  to  engineer  and  geologist,  and 
the  natural  laws  which  govern  them  possess  a  fascinating  in- 
terest for  the  laymen  interested  in  one  of  the  most  impressive 
of  Nature's  destructive  forces. 

Earthquakes  and  explosion  waves  are  comparatively  rare  phe- 
nomena, but  their  spectacular  character,  the  popular  interest 
which  attaches  to  them,  and  the  disasters  for  which  they  are 
responsible,  entitle  them  to  consideration  in  any  treatise  on 
waves.  The  great  wave  known  as  the  "tide  "  is  of  importance 
to  the  student  of  shore  processes  only  in  connection  with  the 
currents  which  it  produces,  and  is  accordingly  given  scant  space 
in  Chapter  I.  A  treatment  of  tidal  currents  will  be  found  in 
Chapter  III.  Standing  waves  including  the  seiche,  and  the 
so-called  ''boundary  waves  "  produced  at  the  contact  of  liquids 
having  different  densities,  are  of  theoretical  rather  than  prac- 
tical interest  in  the  present  connection,  and  are  discussed  very 
briefl}^ 

Scope  of  Subject.  —  Water  waves  may  be  produced  in  a  variety 
of  wa^^s.  The  bow  of  a  vessel  pushing  through  the  water,  or  a 
strong  wind  blowing  over  the  sea,  or  a  rain-drop  falling  into  it,  will 
each  produce  waves;  but  in  each  case  the  waves  are  of  essentially 
different  character,  and  behave  according  to  distinctly  different 
laws.  One  of  the  waves  generated  by  a  submarine  displacement  of 
the  earth's  crust  is  similar  to  the  wave  pushed  out  from  the  bow  of 
a  moving  ship,  but  is  unlike  those  produced  during  a  storm  at  sea. 
Waves  which  form  when  a  fine  wire  is  drawn  through  the  water 
behave  very  differently  from  the  ship's  waves,  but  are  like  the 
ripples  set  in  motion  by  a  falling  drop  of  water.  The  great  wave 
known  as  the  "  tide  "  is  a  compound  wave,  combining  some  of 
the  characteristics  of  the  two  groups  of  waves  first  mentioned. 
When  wind-made  waves  break  on  a  shallow  shore  they  give 
rise  to  a  new  series  of  waves  similar  to  those  produced  by  the 
bow  of  a  vessel.  Such  facts  as  these  are  sufficient  to  show  that 
the  subject  of  waves  is  an  extremely  complicated  one.  Op- 
portunity for  making  direct  observations  of  wave  motion  below 
the  surface  of  water  bodies  in  nature  is  very  limited,  while  the 


SCOPE  OF  SUBJECT  3 

theoretical  treatment  of  wave  motion  carries  one  into  the  reahns 
of  higher  mathematics. 

It  is  not  within  the  scope  of  the  present  report  to  enter  into  a 
discussion  of  all  the  interesting  series  of  water  waves  known  to 
science.  The  beautiful  and  complicated  wave  pattern  developed 
by  a  moving  ship  has  little  to  do  with  the  modelling  of  shore 
forms,  and  the  reader  who  would  follow  that  phase  of  the  sub- 
ject further  is  referred  to  Lord  Kelvin's  popular  lecture  "  On 
Ship  Waves "\  the  second  chapter  of  J.  A.  Fleming's  httle  volume 
on  "  Waves  and  Ripples  "-,  and  the  twelfth  chapter  of  Vaughan 
Cornish's  book  entitled  "  Waves  of  the  Sea  and  other  Water 
Waves  "^,  in  which  latter  place  will  be  found  exquisite  photo- 
graphic illustrations  of  ship  waves.  The  phenomena  of  ripples 
are  treated  at  some  length  by  Fleming*,  and  a  more  technical 
account  is  given  by  J.  Scott  Russell^.  Other  interesting  forms 
of  water  waves  are  described  at  length  by  Russell"  and  Vaughan 
Cornish'^.  We  must  confine  our  discussion  to  those  types  of 
wave  motion  which  have  a  significant  effect  upon  the  shore. 

But  even  if  we  limit  ourselves  to  a  consideration  of  those  waves 
of  practical  importance  to  the  engineer  and  physiographer,  our 
task  is  by  no  means  an  easy  one.  The  literature  of  the  subject  is 
extensive  and  much  of  it  highly  technical  in  character.  Different 
authorities  employ  different  formulae  in  deriving  some  of  the 
elements  of  wave  motion,  and  the  results  they  obtain  agree 
neither  with  each  other  nor  with  the  results  obtained  by  experi- 
mentation. Airy  commends  the  experimental  work  of  J.  Scott 
Russell  as  being  the  best  ever  done,  but  warns  the  reader  against 
accepting  that  author's  theoretical  expressions,  claiming  that 
his  own  formulae  express  the  true  relations  and  are  verified  by 
Russell's  results^  Russell,  in  turn,  demonstrates  the  inaccu- 
racy of  Airy's  formulae,  and  deplores  the  fact  that  the  methods 
of  investigation  employed  by  that  able  authority  should  not  have 
led  him  to  better  conclusions^.  Hagen  likewise  opposes  with 
some  vigor  certain  of  the  suppositions  made  by  Airy,  while  the 
experimental  observations  of  Caligny  and  Russell  disagree  on 
important  points.  Kriimmel  has  well  expressed  the  present  con- 
dition of  the  subject  in  the  words:  "  In  short  analysis,  observa- 
tion and  experiment  are  not  yet  in  the  desired  agreement  "i°. 
Fortunately,  a  numlier  of  the  disputed  points  are  not  of  special 
importance  to  the  student  of  shore  forms,  however  much   he 


4  WATER  WAVES 

may  be  interested  in  the  complex  but  beautiful  laws  which  govern 
the  motions  of  waves. 

Literature. — -Some  of  the  principal  sources  of  information 
upon  which  I  have  relied,  and  to  which  the  student  of  waves  is 
referred  for  elaborate  discussions,  may  briefly  be  mentioned. 
Of  historical  interest  are  the  work  of  Leonardo  da  Vinci,  who  in 
the  latter  part  of  the  fifteenth  century  recognized  many  of  the 
fundamental  principles  of  wave  motion,  and  advanced  theories 
which  are  similar  to  those  of  modern  investigators;  and  of 
Newton,  who  a  century  later  gave  us  the  first  exact  mathematical 
treatment  of  waves.  Among  more  recent  works  the  publications 
of  Franz  Gerstner,  which  appeared  in  the  early  years  of  the  nine- 
teenth century,  are  especially  important.  I  have  not  seen  the 
original  papers  of  these  authors,  l)ut  their  work  is  reviewed  by 
the  Weber  brothers,  Oialdi,  Wheeler  and  others,  in  reports 
mentioned  below. 

In  1809  Bremontier's  able  essay  entitled  ''  Recherches  sur  le 
Mouvement  des  Ondes  "^^  was  published.  This  early  report  of 
experimental  work  on  the  laws  of  wave  action  and  of  observations 
on  wave  action  in  nature,  contains  the  first  effective  demon- 
stration of  the  power  of  waves  to  affect  the  bottom  at  considerable 
depths.  The  important  volume  of  the  two  Weber  brothers  on 
"  Wellenlehre  auf  Experimente  gegrundet  '  ^-,  based  on  elaborate 
experimental  studies  and  published  in  1825,  contains  a  review  of 
practically  everything  written  on  waves  from  the  time  of  Newton 
up  to  1820,  and  adds  much  to  the  sum  of  previous  knowledge 
on  the  subject.  Six  years  later  Emy's  treatise  "  Du  Mouvement 
des  Ondes  et  des  Travaux  Hydrauliques  Maritimes  "^^  refuted 
Bremontier's  conclusion  that  during  wave  movement  the  water 
particles  rose  and  fell  in  vertical  paths,  substituted  the  more 
nearly  correct  opinion  that  the  particles  moved  in  vertical 
ellipses,  and  developed  at  great  length  the  theory  that  a  special 
type  of  "  bottom  wave  "  (flot  de  fond)  was  the  principal  cause 
of  changes  in  the  forms  of  the  coast  and  of  the  destruction  of 
maritime  engineering  structures.  Emy  does  not  appear  to  have 
been  familiar  with  the  work  of  the  Weber  brothers.  J.  Scott 
Russell's  two  reports  on  "  Waves  "i*,  made  to  the  British  Asso- 
ciation in  1837  and  1842-1843,  present  the  results  of  admirable 
experimental  work  made  under  conditions  more  favorable  than 
those  attending  the  experiments  of  the  Weber  brothers,  although 


LITERATURE  5 

Russell  directed  his  attention  principally  to  the  waves  of  trans- 
lation. In  reading  Russell's  reports  the  student  must  guard 
against  misapprehension  arising  from  the  fact  that  the  text 
references  to  plate  numbers  and  to  the  lettering  of  illustrations 
are  full  of  errors.  The  same  author's  great  monograph  on 
"  Naval  Architecture  "^^  contains  several  valuable  chapters  on 
waves.  In  1865  there  were  published  the  results  of  experiments 
made  during  the  preceding  decade  by  Bazin  and  Darcy^''  on  a 
much  more  extensive  scale  than  those  performed  by  Russell. 

Airy's  elaborate  treatise  '"  On  Tides  and  Waves  "^^  appeared 
in  the  Encyclopedia  Metropolitana  in  1845,  and  has  since  been 
recognized  as  the  standard  mathematical  discussion  of  the  theory 
of  waves,  although  the  validity  of  some  of  his  assumptions  has 
been  assailed.  In  spite  of  its  technical  character  the  non- 
mathematical  student  will  find  in  it  much  of  value.  Two  papers 
by  Stokes^^  which  appeared  a  few  years  later  and  which  have 
since  been  included  in  the  first  volume  of  his  "  Mathematical 
and  Physical  Papers,"  are  important  because  of  their  con- 
tributions to  the  theory  of  oscillating  waves.  Rankine  gave  a 
mathematical  analysis  of  the  "  Exact  Form  of  Waves  near  the 
Surface  of  Deep  Water  "^^  in  1863.  Fourteen  years  later  Bous- 
sinesq  produced  his  exhaustive  treatise  entitled  "  Essai  sur  la 
Theorie  des  Eaux  Courantes  "-°  which  includes  an  extended 
mathematical  discussion  of  waves.  Bertin's  long  "  Etude  sur 
la  Houle  et  le  Roulis  "^^  and  still  more  elaborate  "  Donnees 
Theoriques  et  Experimentales  sur  les  Vagues  et  le  Roulis  "^^ 
appeared  in  sections  during  the  decade  1869-1879  in  the  Memoires 
de  la  Societe  Nationale  des  Sciences  Naturelles  de  Cher])ourg,  a 
publication  which  in  the  same  period  carried  articles  on  the  same 
or  related  subjects  by  de  Saint-Venant-^,  Mottez-^  and  others. 
All  of  these  papers  except  the  last  mentioned  are  mathematical 
in  character,  but  contain  matter  of  importance  for  the  non- 
mathematical  student  of  wave  action,  the  later  sections  of 
Bertin's  second  memoir  including  the  results  of  experiments 
made  by  himself  and  Caligny  upon  the  effects  of  waves  breaking 
on  sloping  beaches,  either  with  or  without  the  disturbing  effects 
of  seawalls. 

In  1866  Cialdi  published  his  important  l)ook  "  Sul  INIoto 
Ondoso  del  Mare  e  su  le  Correnti  di  esso"-^  in  which  he  reviews 
the  works  of  many  previous  writers,  particularly  those  of  Italian 


6  WATER  WAVES 

authors,  and  discusses  wave  action  from  the  standpoint  of  the 
engineer.  Caligny's  important  work  on  "  Oscillations  de  I'Eau/'^® 
published  in  1883,  includes  the  results  of  valuable  experimental 
work  on  waves,  particular  interest  attaching  to  his  contributions 
to  our  knowledge  of  waves  of  translation.  Stevenson's  treatise  on 
"  The  Design  and  Construction  of  Harbours  "-^  contains  a  large 
number  of  facts  which  have  materially  increased  our  familiarity 
with  the  mechanical  work  of  waves,  and  from  the  engineering 
point  of  view  is  one  of  the  best  published  treatises  on  wave  action. 
A  little  book  on  "  Waves  and  Ripples  in  Water,  Air  and  Aether  '"^ 
b}'  Fleming,  although  representing  a  course  of  lectures  given 
before  a  juvenile  audience,  presents  in  simple  form  many  laws 
of  wave  motion  which  will  interest  the  older  reader.  Wheeler's 
"  Practical  Manual  of  Tides^and  Waves  "^^  reviews  a  few  of  the 
important  works  on  waves,  and  discusses  the  principles  of  wave 
action  at  some  length.  A  large  number  of  interesting  facts 
concerning  the  behavior  of  waves  will  also  be  found  in  the  same 
author's  volume  on  "  The  Sea  Coast  "^^.  Vaughan  Cornish's 
beautifully  illustrated  book  entitled  "  Waves  of  the  Sea  and 
other  W^ater  Waves  "^^  does  not  consider  the  principles  of  wave 
motion  very  fully,  but  presents  a  wealth  of  facts  concerning  the 
height,  length,  and  other  elements  of  waves,  and  discusses  the 
action  of  waves  on  shore  detritus. 

The  best  general  review  of  the  principles  of  wave  action  which 
has  come  to  my  notice  is  to  be  found  in  the  second  volume  of 
Kriimmel's  "  Handbuch  der  Ozeanographie  "^2.  Gaillard's  trea- 
tise on  "  Wa^^e  Action  in  Relation  to  Engineering  Structures  "^^ 
contains  a  fairly  extended  review  of  the  most  important  work 
of  previous  writers  and  discusses  the  results  of  the  author's  own 
excellent  researches.  The  book  loses  part  of  its  value  as  a  ref- 
erence work  because  many  of  the  quotations  from  the  works  of 
previous  writers  are  unaccompanied  by  such  citations  of  the 
original  sources  as  would  enable  the  reader  to  find  them.  White's 
"  Manual  of  Naval  Architecture  "^"^  has  a  valuable  chapter  on 
deep  sea  waves.  The  numerous  papers  by  Vaughan  Cornish, 
published  in  the  London  Geographical  Journal  and  elsewhere, 
contain  many  interesting  facts  not  stated  in  his  book  above 
mentioned;  and  the  volumes  of  the  "Proceedings  of  the  In- 
stitution of  Civil  Engineers  "  (London)  include  a  number  of 
extended  articles  on  the  action  of  waves  and  currents  upon 


ORIGIN  7 

shore  debris,  which  together  with  the  voluminous  discussions 
appended,  present  various  facts  and  theories  of  interest  to  the 
student  of  wave  action.  Many  other  sources  on  which  I  have 
drawn  are  mentioned  in  the  pages  which  follow. 

WAVES  OF   OSCILLATION 

Origin.  —  The  waves  produced  by  the  action  of  the  wind  are 
the  most  important  type  of  sea  waves.  When  wind  acts  upon  a 
water  surface  it  subjects  it  to  irregular,  unequal  pressure  because 
winds  never  blow  with  constant  velocity,  but  always  in  irregular 
gusts.  Unequal  pressures  deform  the  water  surface,  giving  it 
an  undulatory  form.  The  wind  can  then  act  directly  upon  the 
undulations,  pressing  strongly  against  the  sides  of  the  elevations, 
but  acting  less  effectively  against  the  partially  protected  de- 
pressions. The  water  in  the  elevations  is  moved  forward,  both 
by  direct  pressure  and  by  friction  with  the  passing  air.  This 
action  causes  the  undulations  to  advance  and  to  increase  in  size 
until  the  limit  of  wave  height  for  the  given  wind  velocity  is 
reached,  providing  the  breadth  and  depth  of  the  water  body  are 
sufHciently  great. 

If  one  watches  the  surface  of  a  pond  when  a  faint  breeze 
first  springs  up,  he  will  note  that  the  once  glassy  surface  suddenly 
becomes  covered  with  tiny  ripples,  which  disappear  almost  as 
suddenly  if  the  breeze  dies  down.  But  if  the  breeze  continues, 
it  will  be  seen  that  these  miniature  waves  increase  in  size  pro- 
gressively toward  the  leeward  side  of  the  pond,  those  on  the 
windward  side  remaining  the  original  size.  If  the  breeze  now 
ceases  suddenly,  the  tiny  ripples  on  the  windward  side  quickly 
vanish,  but  the  larger  waves  developed  where  the  wind  blew 
across  a  greater  expanse  of  water  continue  to  agitate  the  surface 
of  the  pond  for  some  time.  It  can  be  shown  that  the  wind  has 
produced  two  distinct  types  of  waves.  The  tiny  ripples  belong 
to  the  class  known  as  capillary  waves,  are  like  the  ripples  pro- 
duced by  a  falling  raindrop  or  a  fine  wire  moved  through  the 
water,  are  due  to  surface  tension  rather  than  to  gravity,  and  move 
the  more  rapidly  the  smaller  the  wave  length.  On  the  other 
hand,  the  larger  waves  on  the  leew^ard  side  of  the  pond  belong 
to  the  class  usually  denominated  by  the  term  "  waves  of  os- 
cillation," are  due_  entirely  to  .S£a\'itj,  move  the  more  rapidly 


8  WATER  WAVES 

the  greater  the  wave  length,  and  very  large  examples  in  the  ocean 
may  travel  for  hours  or  days  without  any  sensible  loss  of  energy 
due  to  viscosity.  There  is  a  certain  length  of  wave,  therefore, 
on  the  border  line  between  large  ripples  and  small  waves  of 
oscillation,  which  has  the  slowest  rate  of  motion.  Progressively 
shorter  waves  travel  with  increasing  velocities  and  belong  to  the 
class  of  ripples.  Those  of  progressively  greater  length  also  travel 
with  increasing  velocities,  but  belong  to  the  class  of  true  waves  of 
oscillation^^.  A  good  brief  summary  of  the  principal  points  in 
the  theory  of  oscillatory  waves  will  be  found  in  a  paper  published 
by  Lyman  in  1868^''. 

Wave  Motion.  —  In  all  types  of  waves,  the  wave  form  moves 
far  over  the  surface  of  the  water  while  the  individual  water 
particles  move  but  a  comparatively  short  distance;  just  as 
"  waves  "  may  be  seen  to  sweep  across  a  wheat-field  with  every 
gust  of  wind,  although  the  individual  stalks  of  wheat  merely 
bend  slightly  and  then  return  to  the'r  original  positions.  The 
contrast  between  wave  movement  and  water  movement  is 
strikingly  exhibited  when  waves  advance  up  an  estuary  during  the 
ebbing  of  the  tide.  In  typical  waves  of  oscillation  in  deep  water 
each  water  particle  moves  through  a  circular  orbit,  the  particle 
moving  forward  on  the  crest  of  the  wave,  downward  on  the  back, 
backward  in  the  trough,  and  upward  on  the  wave  front.  The 
relation  of  the  orbital  paths  of  the  water  particles  to  the  direction 
of  wave  propagation  is  shown  in  Figure  1.  It  is  important  to 
note  with  care  both  the  direction  of  orbital  motion,  and  the  part 
of  the  orbit  in  which  a  water  particle  has  a  given  dh*ection,  as 
these  points  frequently  are  incorrectly  represented.  For  ex- 
ample, one  of  our  best  known  college  texts  on  "  Physiography  " 
contains  a  figure  illustrating  wave  motion  which  erroneously 
shows  the  direction  of  orbital  movement  at  the  crest  of  the  wave 
as  opposite  to  the  direction  of  wave  propagation,  while  the 
black  dots  representing  the  water  particles  are  in  the  wrong 
positions  in  all  of  the  orbits  except  those  showing  the  particle, 
at  the  top  of  wave  crest  and  bottom  of  ^\ave  trough. 

A  cork  or  piece  of  seaweed  floating  on  the  water,  and  moving 
with  the  water  particles,  may  be  seen  to  describe  a  circular  orbit 
when  a  wave  form  passes  under  it.  The  cork  is  at  the  top  of 
its  orbit  as  the  crest  of  a  wave  passes,  reaches  the  bottom  as 
the  trough  passes,   and  attains  the  top ,  when  the  next  crest 


WAVE  MOTION 


arrives.  Thus  the  time  re- 
quired for  the  cork  to  move 
through  its  orbit  is  precisely 
that  required  for  the  crest  of 
the  wave  to  advance  a  dis- 
tance equal  to  one  complete 
wave  length,  i.e.,  the  distance 
from  the  crest  of  one  wave  to 
the  crest  of  the  next.  Now 
in  a  wave  20  feet  high,  having 
a  length  of  1000  feet  or  more, 
it  is  evident  that  the  water 
particle  travels  through  its 
circular  orbit  a  distance  of 
but  little  more  than  60  feet  ^ 
while  the  wave  form  travels 
a  fifth  of  a  mile.  As  we  shall 
see  in  a  later  paragraph,  the 
velocity  of  waves  is  often  so 
great  that  the  ocean  would  be 
unnavigable  were  it  not  for 
the  fortunate  fact  that  the 
water  does  not  travel  with 
the  wave  form. 

Although  emphasis  is  prop- 
erly laid  upon  the  fact  that 
the  particles  of  water  move 
in  a  limited  orbit  while  the 
wave  form  progresses,  the 
common  statement  that  in 
the  open  sea  the  water  parti- 
cles have  no  progressive  mo- 
tion is  not  quite  accurate. 
In  1847  Stokes  demonstrated 
from  the  mathematical  stand- 
point that  "  the  particles,  in 
addition  to  their  motion  of 
oscillation,  will  have  a  pro- 
gressive motion  in  the  direc- 
tion of  propagation  of  the  waves 


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10 


WATER  WAVES 


particles  being  not  altogether  compensated  by  their  backward 
motion.  According  to  Stokes  this  progressive  motion,  in  deep 
water  at  least,  decreases  rapidl}^  as  the  depth  of  the  particle 
considered  increases.  Cialdi  later  discussed  this  progressive  mo- 
tion of  the  water  particles  at  much  length,  and  sought  to  ex- 
plain it  as  in  part  a  consequence  of  the  increase  in  density  of 
the  particles  brought  about  by  the  cooling  due  to  evaporation 
and  radiation  at  the  crests  of  the  waves^'*.  It  is  certain  that 
the  wind  by  pressing  more  upon  the  posterior  parts  of  the  waves 
than  upon  the  anterior  parts,  gives  a  distinct  progressive  motion 
to  the  water  involved  in  oscillatory  waves,  and  that  this  motion 
is  greatest  at  the  surface,  decreasing  with  depth.  Stokes  has 
developed  a  formula  for  calculating  the  extent  to  which  a  ship 
may  be  drifted  from  her  course  b}^  the  progressive  motion  of 
the  water  particles  in  waves  of  this  class,  although  he  does  not 
regard  the  formula  as  of  practical  importances^. 

In  water  of  limited  depth  the  water  particles  move  round 
and  round  in  ellipses  whose  major  axes  are  horizontal  (Fig.  2), 


Fig.  2.  —  Diagram  showing  the  elliptical  orbits  of  water  particles  in  shallow- 
water  waves,  and  the  decrease  in  size  of  orbits  with  increasing  depths. 
(After  Krummel.) 

and  at  the  bottom  the  ellipses  are  reduced  to  straight  lines,  the 
water  particles  simply  moving  forward  and  backward^".  In 
somewhat  deeper  water  the  particles  near  the  surface  will  move 
in  circles,  those  farther  down  in  ellipses,  and  those  on  the  bottom 
in  straight  lines.  It  is  this  back-and-forth  movement  on  the 
bottom  which  Emy*^  was  considering  when  he  proposed  hi& 
theory  of  ''  ground  waves  "  or  "  bottom  waves  "  (flots  de  fond), 
although  he  apparently  included  in  addition  certain  phenomena 
of  waves  of  translation.  This  theory'  was  assigned  an  undue 
importance,  and  was  greatly  elaborated  by  Cialdi'*'  and  Cor- 
naglia^,  and  by  others  of  the  Italian  school  whose  works  discuss 
the  "  flutto  di  fondo  "  at  much  length.     The  latter  author  lays 


WAVE  MOTION  11 

much  stress  on  the  existence  of  a  "  neutral  Une  "  where  the  land- 
ward and  seaward  components  of  the  groundwave  are  supposed 
to  be  exactly  balanced;   and  considered  that  inside  this  line  the 
motion  of  debris  is  landward,  while  outside  it  is  seaward.     Thou- 
let^-i  applies  the  term  "  lames  de  fond  "  to  waves  of  an  entirely 
different  type,  —  waves  originating  from  seismic  disturbances, 
the  discussion  of  which  will  be  taken  up  on  a  later  page.     On 
a  letv^el  sea-bottom  covered  by  a  limited  depth  of  water,  it  is 
evident  that  oscillatory  waves  would  cause  sand  to  shift  back 
and  forth,   but  would  give  to  it  no  progressive  motion,  were 
there  no  progressive  motion  of  the  water  particles  themselves. 
If  we  admit  the  existence  of  the  progressive  motion  discussed  in 
the  preceding  paragraph  as  characteristic  of  normal  waves  of 
oscillation,  it  would  seem  to  follow  that  this  motion  will  still 
obtain  when  the  orbits  are  reduced  to  straight  lines,  and  that  we 
should  therefore  expect,  in  the  absence  of  opposing  forces,  a  slow 
but  progressive  transfer  of  sand  in  the  direction  of  wave  advance. 
Caligny  investigated  a  series  of  waves  formed  by  raising  and 
lowering  a  cyhnder  in  the  end  of  a  wooden  trough,  and  found 
that  the  water  particles  moved  in  elliptical  orbits  which  had  their 
greatest  diameters  vertical  instead  of  horizontal.     It  is  possible 
that  the  orbital  motion  of  this  type  of  wave  is  responsible  for 
those  ihustrations  of  sea  waves  appearing  in  certain  text-books 
of  physical  geography,  in  which  the  orbital  paths  are  shown  as 
eUipses,  with  major  axes  vertical.     But  according  to  Cahgny^^ 
these  waves  are  peculiar  in  several  respects:   they  belong  to  the 
class  of  waves  of  translation,  although  they  have  an  oscillatory 
movement;    and  experiments  showed  that  grains  of  sand  and 
other  material  were  slowly  transported  along  the  bottom  of  the 
trough  in  a  direction  opposite  to  that  of  the  wave  propagation. 
It  would  seem  inadmissible  to  compare  these  waves  with  those 
formed  by  the  wind  in  the  open  ocean.     Bremontier^^  supposed 
that  in  normal  wave  motion  the  water  particles  rose  and  fell  in 
vertical  paths,   while  Emy*^  presents  arguments  to  show  that 
the  paths  must  be  ellipses  with  the  major  axes  vertical.     In 
both  cases  the  arguments  are  evidently  unsound,  and  the  con- 
clusions opposed  by  the  results  of  more  modern  studies  of  deep- 
sea  waves.     In  short,  I  have  not  found  a  satisfactory  basis  for 
those  illustrations  of  deep-sea  waves  showing  elliptical  orbits 
with  major  axes  vertical. 


12 


WATER  WAVES 


As  will  readily  appear  from  Figures  2  and  3  the  size  of  the  orbits 
through  which  the  water  particles  move  decreases  rapidly  with 
increase  in  depth.  At  the  depth  of  one  wave  length  below  the 
surface,  the  water  particles  of  an  oscillatory  wave  are  moving 
in  orbits  whose  diameters  are  only  g^L_  as  great  as  the  diameter 
of  the  orbits  at  the  surface^.     We  may  express  this  relation  in 


Fig.  3.  —  Diagram  showing  theoretical  form  of  a  cycloidal  wave,  and  the 
rapid  decrease  in  size  of  the  orbits  (through  which  the  water  particles 
move)  with  increasing  depth. 


the  following  rule^^:  For  each  additional  ^  of  the  wave  length 
below  the  mid-height  of  the  surface  wave,  the  diameter  of  the 
orbit  is  decreased  by  ^.     Thus: 


Depth  below  mid-height  of  surface  wave  in  frac- 
tions of  wave  length 0, 

Proportionate  diameter  of  orbit 1, 


2)    ?»     8)    T6: 


etc. 
etc. 


For  the  diameter  of  an  orbit  situated  one  wave  length  below  the 
surface,  the  rule  would  give  a  value  of  jy2  of  the  surface  orbit, 
which  is  approximately  correct  and  is  the  figure  quoted  by 
Cornish^°  and  others.  If  the  sea  is  disturbed  by  waves  having 
a  height  of  20  feet  and  a  length  of  400  feet,  the  water  parti- 
cles at  the  surface  move  in  circles  having  a  diameter  of  20  feet, 
while  the  particles  at  a  depth  of  400  feet  move  in  circles  only 
x*o  of  an  inch  in  diameter.  The  importance  of  this  principle  will 
appear  when  we  come  to  consider  the  depths  at  which  waves 
may  erode  the  sea-bottom  and  transport  material. 

Wave  Form.  —  The  theoretical  form  of  oscillatory  waves  in 
the  open  sea  is  indicated  by  Figure  4  which  represents  the  profiles 


WAVE  FORM  13 

of  three  such  waves.  The  profiles  are  trochoidal  curves^S  or  the 
curves  which  would  be  described  by  points  within  a  circle  which  is 
rolled  along  the  under  side  of  a  straight  line.  In  the  figures 
this  curve  is  produced  by  drawing  a  series  of  circular  orbital 


Fig.  4.  —  Theoretical  profiles  of  three  trochoidal  waves  having  different 
sized  orbits  (solid-line  profile  and  broken-line  profile),  or  different  spacing 
of  orbits  (broken-line  profile  and  dotted-line  profile).  Modified  after 
Grabau. 

paths,  indicating  the  proper  position  of  the  water  particle  in 
each  orbit,  and  connecting  these  positions  by  a  curved  line.  As 
will  appear  from  the  figures,  the  sharpness  of  the  wave  crests 
varies  according  as  the  series  of  orbits  having  water  particles 
in  the  same  given  positions,  is  closely  or  widely  spaced.  From 
the  mathematical  standpoint,  the  curve  will  be  sharp  crested  or 
not  according  as  the  point  within  the  rolhng  circle  is  at  or  near 
the  circumference,  or  near  its  center.  If  at  the  circumference, 
the  curve  developed  will  be  the  very  sharp  crested  form  called 
the  cycloid  (Fig.  3).  This  is  the  shortest  and  steepest  form 
which  a  true  wave  theoretically  can  have^-.  As  a  matter  of 
fact  no  wave  approximating  the  form  of  the  common  cycloid 
can  be  produced  in  nature,  as  Gaillard  has  shown^l  In  the 
steepest  deep-sea  waves  observed  the  ratio  of  height  to  length  is 
only  about  one-half  that  demanded  by  the  cycloidal  wave  form^'*. 
It  is  doubtful  whether  the  precise  form  of  the  flatter  trochoid  is 
ever  achieved,  for  it  can  be  shown  that  the  trochoidal  theory  of 
waves  does  not  adequately  satisfy  all  the  conditions  of  wave 
formation^^  Nevertheless,  the  deviation  of  deep-water  waves 
from  the  true  trochoidal  form  is  so  slight,  and  the  trochoidal 
theory,  especially  as  modified  by  Stokes^^  is  so  superior  to  all 
other  theories  of  wave  formation,  that  we  shall  not  go  far  wrong 
if  we  consider  such  waves  as  having  the  form  of  the  trochoid 
and  call  them  trochoidal  waves. 

In  any  trochoidal  wave  the  crest  is  steeper  and  narrower 
than  the  trough  and  contains  an  insufficient  amount  of  water 
to  fill  the  trough.  The  level  of  the  water  during  calm  is  there- 
fore lower  than  the  level  of  the  centers  of  the  orbits  which  the 


14 


WATER  WAVES 


WAVE  FORM  15 

surface  water  particles  describe  during  wave  action.  In  other 
words,  half  the  height  of  the  waves  does  not  give  the  true  sea 
level,  that  level  being  somewhat  lower.  Stevenson  gives  a  for- 
mula prepared  by  Rankine,  for  calculating  the  position  of  mean 
sea  level  when  height  and  length  of  wave  are  known;  he  also 
observes  that  large  waves  in  Wick  Bay  had  about  two-thirds 
of  their  height  above  still-water  level,  and  one-third  below^^. 
On  the  basis  of  extensive  observations  Gaillard  has  devised 
more  satisfactory  formulse  for  determining  the  still-water  level, 
taking  due  account  of  the  fact  that  a  larger  percentage  of  wave 
height  is  above  still-water  level  in  shallow  water  than  would  be 
indicated  by  a  formula  which,  like  Stevenson's,  is  applicable  to 
deep-water  waves.  Gaillard  found  that  in  shallow  water  about 
three-quarters  of  the  wave  height  is  above  still-water  level 
just  before  the  wave  breaks'^'^  The  importance  of  this  fact  will 
be  apparent  when  it  is  remembered  that  the  effective  salt-water 
level  of  the  sea  may  thus  be  raised  a  number  of  feet  above  high 
tide  level,  and  also  that  floating  logs  or  blocks  of  ice  may  accom- 
plish considerable  work  to  any  height  reached  by  the  crest  of 
the  waves. 

When  a  strong  wind  is  blowing,  the  trochoidal  profile  of  the 
waves  is  seen  to  be  materially  altered.  If  the  wind  is  in  the 
direction  of  wave  propagation,  as  is  more  commonly  the  case  in 
the  open  sea,  the  forward  motion  of  the  water  particles  on  the 
wave  crest  is  accelerated,  while  the  backward  motion  in  the 
trough  is  retarded.  Since  the  troughs  are  somewhat  protected 
from  the  wind,  the  retardation  is  less  effective  than  the  accelera- 
tion of  the  wave  crests.  The  net  result  is  a  steepening  of  the 
front  of  the  wave,  so  that  the  profile  becomes  noticeably  asym- 
metrical. Winds  of  sufficient  velocity  may  even  force  some  of 
the  water  on  the  wave  crest  out  of  its  orbital  path,  blowing  it 
forward  into  the  adjacent  trough  in  the  form  of  foam  and  spray. 
When  asymmetrical  waves  pass  out  of  the  region  of  the  storm 
winds  which  generated  them,  they  decrease  in  height,  become 
more  rounded  and  symmetrical,  and  closely  approach  the  tro- 
choidal form,  although  the  steeper  front  has  been  observed  on 
deep-water  waves  in  calm  weather^^.  These  waves  may  be 
propagated  hundreds  or  thousands  of  miles  from  the  storm  center 
where  they  originated,  and  ultimately  become  the  gentle  undu- 
lations known  as  the  ''  swell,"  or  "  ground  swell." 


16  WATER  WAVES 

Surf.  —  A  very  important  alteration  of  form  occurs  when  the 
oscillatory  wave  passes  into  shallow  water.  The  wave  becomes 
higher  and  shorter,  the  front  steepens,  the  crest  arches  forward 
and,  finding  itself  unsupported  by  sufficient  water  on  the  front  of 
the  wave,  dashes  downward  with  a  roar,  producing  the  phenom- 
enon known  as  the  "  surf."  An  individual  breaking  wave  is 
known  as  a  ''  breaker,"  or  less  frequently  as  a  "  combing  wave  "; 
the  latter  term  is  also  applied  to  a  deep-water  wave  whose  crest  is 
pushed  over  forward  by  a  strong  wind.  The  commonly  accepted 
explanation  of  surf  is  that  the  wave  is  retarded  by  friction  when 
it  enters  shallow  water,  the  lower  part  "  dragging  "  on  the 
bottom  while  the  upper  part  advances  unimpeded,  until  the  wave 
becomes  so  steep  in  front  that  it  falls  forward.  There  seem  to  be 
fatal  objections  to  this  theory  of  surf  action.  In  the  first  place 
the  amount  of  friction  necessary  to  produce  the  observed  result 
does  not  seem  to  exist.  Experimental  studies  of  waves  in  shallow 
water  of  uniform  depth  under  conditions  favorable  for  the 
development  of  frictional  retardation  fail  to  show  it^".  On  the 
other  hand,  it  will  later  be  shown  that  wave  velocity  decreases 
with  decreasing  depth.  It  is  equally  certain  that  the  size  of 
the  orbital  paths  increases  as  waves  enter  shallower  water, 
while  at  the  same  time  the  volume  of  water  is  decreasing.  With 
constantly  enlarging  orbits  and  diminishing  water  supply,  there 
must  come  a  time  when  the  volume  of  water  is  insufficient  to 
build  up  the  entire  wave  form,  the  deficiency  manifesting  itself 
as  a  "  hollowing  "  of  the  front  of  the  wave.  The  water  available 
endeavors  to  curve  around  through  the  entire  orbit,  but  on  reach- 
ing the  top  of  the  circle  finds  itself  unsupported  and  collapses. 

The  form  of  a  breaking  wave  is  not  that  which  should  exist 
if  friction  were  the  principal  cause  of  the  surf.  If  the  observer 
can  secure  a  position  where  the  wave  profile  is  discernible,  he 
will  find  that  there  is  a  steepening  of  the  wave  front,  to  be  sure, 
but  the  form  does  not  suggest  a  steepening  due  to  forward  in- 
clination of  the  whole  wave  mass  resulting  from  "  bottom  drag/' 
so  much  as  it  does  a  steepening  due  to  the  absence  of  water  on, 
and  consequent  hollowing  of  the  front  side  of  the  wave.  When 
the  wave  finally  breaks,  masses  of  foam  floating  on  the  water 
surface  appear  to  describe  an  orbit  that  is  more  symmetrical 
than  one  should  expect  in  a  wave  deformed  by  great  bottom 
friction,   while  the  forward  arching  crest  tries  to  complete  a 


WAVE  FORM 


17 


:3 


fcJD 

a 

o 


18  WATER  WAVES 

wave  form  which,  if  achieved,  would  not  show  excessive  steepen- 
ing on  the  front.  (Plate  II.)  The  credit  for  first  stating  the 
above  explanation  of  surf  action  belongs  to  Hagen^^. 

Depth  at  Which  Waves  Break.  —  The  depth  of  water  in  which 
the  oscillatory  wave  assumes  the  form  of  a  breaker  is  a  mat- 
ter of  some  interest.  As  in  the  case  of  the  wave  of  translation, 
described  below,  Russell'^-  found  that  breaking  occurred  when 
the  depth  of  the  water  equalled  the  height  of  the  wave,  a  rule  not 
wholly  confirmed  b}^  the  experiments  of  Bazin*'^,  who  found  that 
breaking  occurred  more  frequently  when  the  height  of  the  wave 
exceeded  two-thirds  of  the  total  depth.  Russell  states  that  his  rule 
also  holds  good  for  oscillatory  waves,  but  unfortunately  he  is 
neither  clear  nor  consistent  in  his  method  of  calculating  wave 
height  and  water  depth  in  the  case  of  these  waves.  In  one  place 
we  rea.d  that  "  every  wave  broke  exactly  when  its  height  above  the 
antecedent  hollow  was  equal  to  the  depth  of  the  water,"  the 
method  of  calculating  water  depth  not  being  stated;  on  another 
page  both  wave  height  and  water  depth  are  apparently  mea- 
sured from  mean  water  level;  according  to  a  third  statement  the 
author  never  saw  a  wave  as  much  as  10  feet  high  in  10  feet  of 
water,  nor  20  feet  high  in  20  feet  of  water,  although  he  has  seen 
waves  approach  very  nearly  to  those  limits'''*.  Cornish  expresses 
the  rule  as  follows:  waves  break  when  the  depth  of  water  reck- 
oned from  the  undisturbed  sealevel  is  equal  to  the  height  of  the 
crest  above  the  trough^\  In  other  words,  a  wave  entering 
shallowing  water  increases  in  height  as  the  water  decreases  in 
depth  until  the  height  of  the  wave  above  the  trough,  and  the  mean 
water  depth  reach  approximate  equality,  when  the  wave  breaks. 
According  to  this  rule  the  navigator  who  sees  waves  8  or  9  feet 
high  (or  about  6  feet  above  still-water  level)  breaking  over  a 
certain  submarine  bar,  may  know  that  he  can  count  on  but  8 
or  9  feet  of  mean  water  depth,  or  6  feet  of  depth  below  the  trough, 
at  the  place  in  question.  Some  other  factor  or  factors,  however, 
combine  with  water  depth  to  determine  the  breaking  of  a  wave, 
with  the  result  that  the  above  rule  does  not  always  hold.  De- 
partures from  the  rule  are  noted  by  Stevenson^^.  Cialdi^^  cites 
a  great  number  of  cases  in  which  waves  have  been  known  to 
break  in  water  many  times  deeper  than  the  wave  height,  and 
both  Thoulet^^  and  KriimmeP^  have  placed  some  of  these  in 
tabular  form.     The  latter  author  suggests  that  the  frequent 


WAVE  FORM 


19 


20  WATER  WAVES 

breaking  of  waves  in  deep  water  just  above  the  outer  edge  of  a 
submarine  terrace  may  be  due  to  an  upward  push  imparted  to 
the  lower  water  when  it  comes  against  the  terrace  face,  this 
push  being  transmitted  to  the  surface  and  causing  the  waves 
to  break^".  Gaillard  found  that  while  oscillatory  waves  some- 
times break  quite  uniformly  when  the  true  height  of  the  wave 
equals  the  depth  of  the  water  measured  from  still-water  level, 
in  other  cases  they  break  when  the  ratio  of  water  depth  to  wave 
height  is  from  1.16  to  2.71.  He  observed  that  the  depth  at 
which  breaking  occurs  varies  with  variations  in  wind  velocity, 
slope  of  bottom,  smoothness  of  bottom,  and  wave  length;  and 
suggests  that  the  strength  of  the  undertow  is  probably  another 
important  factor  in  determining  the  depth  at  which  waves  break. 
In  addition  to  his  own  observation  Gaillard  quotes  those  of 
many  other  observers''^  The  depth  of  breaking  is  of  importance 
in  determining  the  position  of  barrier  beaches  and  other  related 
shore  forms. 

Intersecting  Waves.  —  Thus  far  we  have  considered  the  form  of 
waves  from  the  standpoint  of  changes  in  profile.  If  now  we  turn 
to  their  variations  in  form  along  the  crest  line,  we  have  first  to 
note  that  the  typical  oscillatory  wave  can  not  be  traced  far  in  the 
direction  indicated.  The'crest  soon  descends  at  either  end  and  is 
lost  in  the  maze  of  other  waves.  In  the  open  sea  one  experiences 
the  greatest  difficulty  in  determining  the  end  limits  of  a  given 
crest,  and  also  in  following  the  progressing  crest  for  any  length 
of  time.  The  reason  for  this  is  found  in  the  fact  that  more  than 
one  set  of  waves  are  always  disturbing  the  ocean  surface,  and 
the  several  sets  intersect  each  other  at  various  angles.  Even 
with  two  intersecting  series  it  is  evident  that  the  water  will 
rise  very  high  where  crest  coincides  with  crest,  will  fall  very 
low  where  trough  coincides  with  trough,  and  will  have  all  in-? 
termediate  elevations  where  different  parts  of  the  front  and  back 
of  one  wave  intersect  different  parts  of  another  wave.  Imagine 
several  series  of  waves  crossing  each  other  at  distinctly  different 
angles,  and  we  have  an  adequate  explanation  for  all  the  great 
irregularity  in  wave  form  observed  in  the  open  ocean.  Only 
when  the  observer  is  stationed  high  above  the  tossing  waters, 
and  then  only  under  favorable  conditions,  can  he  distinguish  the 
several  orderly  systems  of  waves  which  are  responsible  for  the 
apparent  chaos. 


WAVE  HEIGHT  21 

But  even  in  a  single  wave  system  the  crests  are  not  of  in- 
definite extent.  Tiiis  is  because  the  wind  which  causes  the  waves 
is  never  of  uniform  strength,  and  because  the  large  waves  result 
in  part  from  unequal  combinations  of  smaller  waves,  as  shown  on 
a  later  page.  The  wind  comes  in  gusts  of  varying  strength  and 
somewhat  varying  direction,  and  so  irregular  a  force  could  not 
produce  a  regular  wave  crest  stretching  far  over  the  ocean. 
Instead  we  have  a  large  number  of  short,  nearly  parallel,  over- 
lapping crests  which  in  course  of  time  combine  into  a  smaller 
number  of  larger  but  decidedly  irregular  waves.  Even  in  the 
region  of  the  trade  winds,  where  the  winds  blow  with  an  un- 
usual degree  of  regularity,  "  the  open  sea  does  not  present  a 
series  of  parallel  ridges,  each  one  of  uniform  height,  with  a  lat- 
eral extension  many  times  greater  than  the  distance  from  crest 
to  crest  "^2.  On  the  contrary,  there  is  no  evidence  of  any  contin- 
uous approximation  toward  regularity. 

Wave  Height.  — ^  In  discussing  the  sizes  of  waves  we  have  to 
do  with  two  principal  elements  of  wave  form:  the  height  measured 
from  the  bottom  of  the  trough  to  the  top  of  the  crest;  and  the 
le?igth  measured  from  crest  to  crest,  or  from  trough  to  trough. 
The  initial  height  of  the  oscillatory  waves  depends  on:  (1)  the 
strength  of  the  wind,  (2)  its  duration,  and  (3)  the  extent  of  open 
water  over  which  it  blows.  A  faint  breeze  sets  in  motion  very 
small  waves  which  increase  in  size  to  a  certain  limit,  but  which 
would  never  become  great  billows.  In  the  trade  wind  belt  the 
maximum  height  of  wave  for  a  certain  strength  of  wind  is  soon 
reached,  and  although  the  wind  may  continue  steadily  for  days 
at  the  given  strength,  there  is  no  increase  in  the  size  of  the  waves. 
In  a  general  way,  the  velocity  of  the  wind  in  statute  miles  per  hour 
divided  by  2.05  will  give  the  height  of  the  waves  in  ieeiP.  Thus 
the  average  height  of  waves  in  a  gale  blowing  44  statute  miles  per 
hour  is 

44  -^  2.05  =  21.5  feet. 

It  should  be  noted,  however,  that  in  very  severe  storms  the 
highest  waves  may  not  occur  when  the  wind  velocity  is  at  a 
maximum,  but  are  seen  to  develop  as  the  wind  begins  to  subside. 
The  explanation  of  this  phenomenon  is  probably  to  be  found  in 
the  fact  that  the  excessive  force  of  a  violent  wind  blows  off  the 
tops  of  the  waves  and  casts  them  into  the  preceding  troughs, 


22  WATER  WAVES 

thereby  materially  diminishing  the  wave  height.  It  is  possible 
also  that  as  the  storm  subsides  the  waves,  which  were  com- 
pelled to  remain  independent  and  irregular  under  the  gusty  force 
of  the  storm  wind,  gradually  combine  into  a  smaller  number  of 
larger  waves  which  arc  little  affected  by  the  failing  strength  of 
the  dying  wind^^. 

Effect  of  Wind  Duration.  —  Wind  duration  is  another  factor  in 
increasing  wave  height  up  to  the  limiting  height  for  a  given  wind 
strength.  When  a  breeze  springs  up,  small  ripples  first  appear 
over  the  water  surface,  but  gradually  develop  to  larger  size  with- 
out any  increase  in  the  strength  of  the  breeze.  If  a  large  swell  is 
already  running  in  the  direction  of  the  wind,  a  sudden  increase  in 
wind  velocity  results  in  increased  height  of  waves;  but  in  this  case 
the  wind  does  not  have  to  endure  very  long  to  bring  about  a  very 
remarkable  increase  in  height.  Cornish  has  recorded  an  increase 
of  7  feet  in  the  height  of  waves  during  a  squall  lasting  4  minutes, 
and  an  increase  of  2  feet  per  minute  in  the  height  of  waves  during 
another  squalF^.  The  precise  method  bj^  which  small  wind  waves 
grow  to  large  ones  is  not  wholly  understood,  but  the  Weber 
brothers  give  the  following  four  causes  for  wave  enlargement: 
(1)  the  continuous  horizontal  pressure  of  the  wind  upon  the 
wave  crest,  thus  tending  to  enlarge  the  orbital  movement  of  the 
water  particles;  (2)  the  combining  of  several  smaller  waves 
moving  in  the  same  direction ;  (3)  the  pressure  exerted  by  a  large 
wave  upon  the  next  following  smaller  wave,  by  which  the  latter 
is  enlarged;  and  (4)  the  crossing  of  waves  proceeding  in  different 
directions''".  Cornish  thus  states  another  theory  of  wave  en- 
largement: "  The  horizontal  velocity  of  the  air  being  greatest 
at  the  crest,  the  downward  pressure  of  the  atmosphere  is  least 
there.  Conversely  at  the  trough,  where  horizontal  velocity  is 
least,  downward  pressure  is  greatest.  Hence  the  trough  is 
pushed  farther  down  and  the  crest  is  sucked  up  "". 

Effect  of  Length  of  Fetch.  —  Of  corresponding  importance  is  the 
effect  of  "  length  of  fetch  "  of  the  wind  across  open  water  upon 
wave  height.  We  have  already  seen  that  when  a  breeze  blows 
across  a  pond  there  first  appear  small  ripples  over  all  its  surface 
but  thatj  these  soon  increase  in  size  progressively  toward  the 
leeward  side  of  the  pond.  The  ripples  on  the  windward  side, 
where  the  wind  has  blown  across  a  small  expanse  of  water  only, 
remain  small  no  matter  how  long  or  how  strong  the  breeze  may 


WAVE  HEIGHT 


23 


blow.  But  those  on  the  leeward  side,  where  the  fetch  of  the 
wind  across  open  water  is  greater,  soon  develop  into  waves  of 
some  size  because  here  the  waves  due  to  the  direct  effect  of  the 
wind  are  combined  with  the  waves  originating  on  the  opposite 
side  of  the  pond  and  propagated  by  gravity  in  the  direction  of  the 
wind.  This  illustrates  on  a  small  scale  a  matter  of  much  impor- 
tance in  the  case  of  sea  waves.  Stevenson  has  shown  that  for 
ordinary  gales  and  distances  the  height  of  the  waves  in  feet  is 
1.5  times  the  square  root  of  the  distance  in  nautical  miles  which 
the  wind  has  blown  over  open  water^^;   or 


height  =  1.5  Vdistance. 

Gaillard  observed  waves  23  feet  high  near  Duluth  with  a 
length  of  fetch  of  259  nautical  miles''^.  This  agrees  fairly  well 
with  the  calculated  height  of  24.1  feet  based  on  the  formula. 
Des  Bois  prepared  a  table  to  show  the  heights  of  waves  corre- 
sponding to  different  wind  velocities,  based  on  his  observation 
that  a  wave  2  meters  high  corresponded  to  a  wind  velocity  of 
5  meters  per  second,  and  the  provisional  theory  that  "  the  square 
of  the  velocity  of  the  wind  will  be  proportional  to  the  cube  of 
the  height  of  the  wave";  and  he  found  that  this  table  corre- 
sponded roughly  with  the  results  he  obtained  from  a  large  number 
of  direct  measurements^". 

For  short  distances  a  modification  of  Stevenson's  formula 
is  necessary.  The  following  table  is  condensed  from  one  given 
by  that  author,  and  shows  the  appproximate  heights  of  waves 
as  determined  by  length  of  fetch,  assuming  great  depth  of  water 
and  a  strong  gale  of  wind. 

TABLE  SHOWING  APPROXIMATE   HEIGHTS  OF  WAVES 
DUE  TO   DIFFERENT   LENGTHS  OF  FETCH 


Nautical 

Heights 

Nautical 

Heights 

Nautical 

Heights 

miles 

in  feet 

milea 

in  feet 

miles 

in  feet 

1 

3 

5 

4.3 

50 

10.6 

2 

3.4 

10 

5.6 

100 

15 

3 

3.8 

20 

7.1 

200 

21.2 

4 

4.1 

30 
40 

8.4 
9.5 

300 

26 

For  expanses  of  open  water  exceeding  500  or  600  miles  in 
length  the  height  of  storm  waves  does  not  appear  to  increase 


24  WATER  WAVES 

according  to  Stevenson's  empirical  formula.  With  a  fetch  of 
3600  miles  the  waves  should  reach  a  height  of  90  feet,  but  so 
great  a  height  is  probably  never  attained.  The  reason  for  this 
discrepancy  is  doubtless  to  be  found  in  the  fact  that  we  have  no 
storm  winds  blowing  steadily  for  a  long  period  in  the  same  di- 
rection over  so  great  a  stretch  of  water^K  The  facts  that  the  wind 
direction  may  be  approximately  the  same  over  a  long  stretch  of 
water,  or  that  it  may  have  a  constant  direction  for  several  days 
at  a  given  place,  as  noted  by  Redfield  and  by  Stevenson*^^  are 
not  alone  sufficient.  The  winds  must  blow  with  the  strength 
of  a  strong  gale  in  a  constant  direction  over  the  entire  distance 
for  several  days,  if  the  full  effect  of  a  2000  or  3000  mile  "fetch" 
is  to  be  realized,  since  the  waves  formed  to  windward  must 
have  time  to  travel  the  long  distance  to  leeward  and  produce  the 
cumulative  effect  which  results  in  maximum  wave  height.  In  our 
cyclonic  storms  the  greatest  distance  traversed  by  heavy  winds 
in  a  reasonably  constant  direction  and  for  a  period  of  time 
sufficient  for  large  waves  to  develop,  probably  does  not  exceed 
600  or  700  miles.  The  "  effective  fetch,"  therefore,  is  much 
more  limited  than  the  absolute  distance  across  open  water;  and 
Vaughan  Cornish  has  estimated,  from  a  study  of  charts  illus- 
trating weather  conditions  in  the  North  Atlantic  Ocean  for 
nine  weeks  of  exceptionall}'  stormj^  weather,  that  the  greatest 
effective  length  of  fetch  during  that  period  was  about  600  nautical 
miles^.  But  while  waves  formed  on  greater  expanses  of  open 
water  do  not  reach  the  heights  calculated  from  the  formula 
given  above,  they  do  exceed  the  altitude  of  about  37  feet  cal- 
culated for  the  greatest  effective  fetch,  because  they  may  com- 
bine with  an  already  existing  swell  and  thereby  increase  their 
height. 

Recorded  Wave  Heights.  —  Observations  of  the  heights  of  waves 
are  often  unreliable,  but  the  approximate  height  under  different 
conditions  has  been  pretty  well  established  by  a  number  of  compe- 
tent observers.  On  Lake  Superior,  waves  reach  a  height  of  from  20 
to  25  feet^;  in  the  Mediterranean  Sea,  25  to  30  feet^^  Scoresby's 
oft-quoted  observations  on  the  North  Atlantic  give  a  height  of  43 
feet  for  the  largest  waves^^  and  Cornish  reports  waves  43  feet  high 
from  the  same  ocean^''.  When  two  great  waves  intersect,  peaks 
of  water  may  rise  momentarily  to  a  height  of  50  or  even  60  feet^^ 
Although  the  North  Pacific  Ocean  has  a  breadth  of  open  deep  sea 


WAVE  HEIGHT  25 

much  greater  than  that  of  the  North  Atlantic,  the  waves  do  not 
appear  to  reach  any  greater  height*^;  but  in  the  Southern  Ocean 
waves  attain  heights  of  from  45  to  50  feet^°.  White  refers  to 
trustworthy  observations  of  waves  of  a  single  series  having 
heights  of  44  to  48  feet,  and  mentions  waves  formed  by  the  com- 
bination of  two  or  more  series  said  to  attain  from  58  to  65  feet^^ 
Gaillard  gives  an  interesting  tabulation  of  the  height,  length, 
and  period  of  ocean  waves  recorded  by  a  number  of  different 
observers,  the  highest  figure  for  wave  height  in  the  table  being 
"  greater  than  50  feet,"  in  the  case  of  a  wave  photographed  by 
Capt.  Z.  L.  Tanner  of  the  U.  S.  Navy^^  By  means  of  a  barom- 
eter Abercromby  measured  waves  46  feet  high  in  the  Southern 
Ocean,  and  concluded  that  some  waves  certainly  attain  a  height 
of  60  feet^^.  Airy  was  of  the  opinion  that  under  no  circum- 
stances does  the  height  of  an  unbroken  wave  exceed  30  or  40 
feet^"*;  but  against  this  theoretical  opinion  we  may  safely  accept 
the  figures  of  competent  observers,  and  conclude  that  waves 
40  feet  high  are  of  fairly  frequent  occurrence  in  the  open  ocean, 
while  heights  of  50  feet  or  more  are  rare,  but  not  unknown. 

When  these  high  storm  waves  run  out  of  the  storm  area,  they 
gradually  decrease  in  altitude,  and  in  the  form  of  swells  usually 
do  not  exceed  a  height  of  15  or  20  feet.  By  the  time  they  are 
nearmg  a  distant  coast  they  may  have  been  reduced  to  heights 
of  a  few  feet  only,  and  so  have  become  almost  imperceptible. 
Entermg  shallowing  water  they  seem  to  awaken  to  new  life, 
crowding  closer  together  and  increasing  in  height  until  they  break. 
At  the  time  of  breaking  the  wave  height  may  be  anywhere  from 
a  few  feet  up  to  25  feet  or  more.  If  a  wave  oomes  in  contact  with 
a  vertical  wall  or  cliff  the  base  of  which  reaches  down  to  deep 
water,  the  wave  is  reflected  back  without  breaking.  The  water 
next  the  wall  moves  up  and  down  through  a  vertical  distance 
equal  to  twice  the  original  height  of  the  wave,  as  does  also  the 
water  half  a  wave  length  from  the  wall.  Similarly,  a  wave  run- 
ning in  a  direction  parallel  to  a  vertical  or  steep  wall  has  that 
portion  of  the  wave  next  the  wall  notably  increased  in  height^^. 

Combined  Waves.  —  Waves  which  appear  to  belong  to  the  same 
series  vary  greatly  in  height.  The  larger  figures  given  above  are  for 
individual  waves,  and  in  each  case  the  average  height  for  the  series 
to  which  the  waves  belonged  was  much  less.  Thus  a  wave  40  feet 
high  may  occur  in  a  series  of  waves  having  an  average  height  of 


26 


WATER  WAVES 


but  20  or  25  feet^''.  This  inequality  in  wave  height  is  probably  due 
in  considerable  part  to  the  fact  that  what  appears  to  be  a  single 
series  of  waves  of.  irregular  height  is  really  the  combined  effect  of 
two  or  more  series  of  waves  moving  intlie  same  direction,  each  series 
having  different  but  fairly  constant  height  and  length.  Figure  5 
from  Cornish's  work  on  "  Waves"  shows,  in  the  third  line,  the  pro- 
file of  an  apparently  irregular  series  of  waves  (c)  resulting  from  the 
combination  of  the  two  regular  series  (a  and  h)  shown  in  the  first 
and  second  lines.  By  holding  the  page  with  the  figure  nearly  on  a 
level  with  the  eye,  but  slightly  inclined  toward  the  observer,  the 
marked  irregularity  of  the  combined  series  may  easily  be  detected. 


Fig.  5.  —  Diagram  showing  how  two  regular  series  of  waves  (a  and  b)  of 
different  heights  and  lengths  combine  to  form  an  irregular  series  (c). 

The  successive  wave  heights,  in  feet,  measuring  from  each  crest 
to  the  next  trough,  toward  the  right,  are  as  follows:  22.50, 
37.50,  18.75,  40.00,  27.50.  The  average  wave  height  for  an 
indefinite  length  of  this  irregular  series  will  be  30  feet  or  pre- 
cisely the  height  of  the  dominant  regular  series.  It  is  evident, 
therefore,  that  an  observer  might  conclude  that  the  sea  was 
disturbed  by  a  single  series  of  waves  600  feet  long  and  30  feet 
in  average  height,  and  that  the  real  presence  of  a  swell  20  feet 
high  would  be  undetected.  This  may  explain  the  fact  that 
during  a  storm  at  sea  the  long  swell  remains  invisible,  yet  be- 
comes noticeable  as  soon  as  the  shorter  storm  waves  die  down 
a  little^^.  Gaillard  suggests,  however,  that  the  waves  first  made 
by  a  strong  wind  are  of  unstable  form  and  cannot  travel  far 
without  being  destroyed  and  contributing  their  energy  to  the 
more  stable  waves  of  nearly  perfect  trochoidal  form,  the  "  swell "; 
while  Tajdor  is  of  the  opinion  that  direct  wind  action  causes  the 


WAVE  LENGTH  27 

water  particles  to  move  in  orbits  of  varying  amplitude  and 
velocity,  producing  a  confused  sea;  but  that  as  soon  as  the 
wind  ceases  the  viscosity  of  the  water  tends  to  make  the  orbits 
identical,  and  thus  to  produce  a  more  uniform  system  of  tro- 
choidal  waves^^. 

The  combination  of  two  or  more  series  of  waves  moving  in 
the  same  direction  explains  the  fact  that  when  waves  break  upon 
the  shore,  there  is  a  recurrence,  at  intervals,  of  waves  of  excep- 
tional height.  It  should  be  noted,  however,  that  while  the 
popular  idea  that  every  seventh  wave  is  a  big  one  rests  upon  a 
basis  of  fact,  the  ratio  of  wave  lengths  in  the  combining  series 
is  just  as  likely  to  make  every  third,  or  ninth,  or  some  other 
wave  the  largest;  or  if  three  sets  of  waves  combine,  the  large 
waves  may  arrive  at  irregular  intervals. 

Wave  Length.  —  The  total  energy  of  a  wave  has  been  shown 
to  vary  nearly  as  the  square  of  the  height  and  as  the  first  power 
of  the  length,  so  that  these  dimensions  may  be  said  to  measure 
the  capacity  of  a  wave  for  destruction'''-*.  Of  these  two  important 
elements  of  wave  form  we  have  just  considered  the  height,  and 
may  now  turn  our  attention  to  the  length. 

The  ratio  of  wave  height  to  wave  length  is  a  matter  of  con- 
siderable interest.  Inasmuch  as  storm  waves  usually  appear 
higher  and  steeper  than  those  in  a  moderate  sea,  we  should 
expect  this  ratio  to  increase  with  increasing  roughness  of  the  sea. 
Lieutenant  Paris  found  that  in  a  light  sea  the  ratio  of  height  to 
length  is  only  1  to  39,  in  a  rough  sea  1  to  21,  while  in  a  heavy  sea 
it  rises  to  1  to  19""'.  Schott  compared  the  ratios  directly  with 
the  strength  of  the  wind,  and  found  that  with  a  moderate  wind 
the  ratio  of  height  to  length  was  1  to  33,  with  a  strong  wind  1 
to  18,  and  with  a  storm  wind  1  to  17  or  even  as  high  as  1  to  13^°^ 
On  thu  other  hand,  White  compares  the  ratios  with  the  lengths 
of  the  waves,  and  shows  that  as  the  lengths  increase  the  ratios 
diminish.  Thus  he  finds  from  an  analysis  of  179  published 
French  observations  that  with  a  wave  length  of  less  than  100 
feet,  the  average  ratio  of  height  to  length  is  1  to  17;  with  a 
length  of  100-200  feet,  the  ratio  is  1  to  20;  with  a  length  of 
200-300  feet,  1  to  25;  with  a  length  of  300-400  feet,  1  to  27.  For 
greater  wave  lengths  the  figures  are  not  wholly  in  accord  with 
the  theory,  while  in  waves  from  100  to  400  feet  long  the  very 
small  ratio  of  1  to  50  has  been  observed^"-.     Cornish  has  com- 


28  WATER  WAVES 

pared  the  lengths  of  waves  with  the  expanse  of  open  water  over 
which  the  wind  blows  and  finds  that  "  the  length  o-f  the  storm- 
waves  is  increased  when  the  length  of  the  sheet  of  water  is 
increased,  but  more  slowly  "^^^ 

The  lengths  of  deep-water  waves  are  quite  definitely  related 
to  their  velocities  and  to  their  periods,  as  will  be  shown  more  fully 
on  a  later  page;  but  we  may  note  here  that  the  wave  length 
(in  feet)  is  roughly  equal  to  5|  times  the  square  of  the  period 
(in  seconds).  Thus,  if  waves  pass  a  given  point  at  the  rate  of 
one  in  every  4  seconds,  the  wave  length  must  be  approximately 

82  feet;  for 

Length  =  5|  (period)^ 

=  51  (4)2 
=  82  feet. 

Recorded  Wave  Lengths.— The  greatest  trustworthy  measurement 
of  wave  length  is  that  recorded  by  Capt.  Mottez  of  the  French 
Navy,  for  a  wave  in  the  North  Atlantic,  measuring  2750  feet  from 
crest  to  crest.  In  the  English  Channel  Cornish  observed  waves 
whose  period  indicated  a  length  of  2594  feeti^".  Ross  observed  a 
wave  in  the  South  Atlantic  1920  feet  longio^.  The  greatest  length 
reported  by  Des  Bois  is  1640  feet^^^,  while  Major  Leonard  Darwin 
found  the  waves  of  an  exceptionally  severe  storm  in  the  Southern 
Ocean  to  be  1200  feet  in  lengthi^^.  go^^g  of  these  high  figures  are 
probably  due  to  the  combination  of  two  sets  of  waves  in  such  a 
manner  as  to  give  an  abnormally  long  stretch  of  low  water  between 
two  crests,  for  storm  waves  in  the  open  sea  are  not  usually  more 
than  600,  and  very  rarely  more  than  700  feet  long.  Scoresby 
found  the  extreme  length  of  the  great  storm  waves  measured  by 
him  to  be  790  feeti°'l  Officers  on  the  North  Atlantic  liners  regard 
600  feet  as  an  enormous  wave  length,  although  they  agree  that 
larger  lengths  are  to  be  found  in  the  Southern  Ocean,  where  in 
one  exceptional  storm  Lieutenant  Paris  found  the  greatest  aver- 
age length  was  771  feet,  with  not  a  few  waves  over  900  feet,  and 
several  surpassing  1312  feet  in  length^'^^ 

,  There  seems  to  be  little  doubt,  however,  that  the  swell  has 
a  length  often  more  than  double  that  of  storm  waves,  and  at 
least  one  of  the  figures  given  above,  that  of  2594  feet  for  the 
length  of  waves  observed  by  Cornish,  refers  to  the  swell.  When 
the  swell  enters  shallow  water  the  velocity  and  wave  length  are 


WAVE  VELOCITY  29 

diminished,  but  the  period  remains  the  same.  Since  the  period 
bears  a  definite  relation  to  the  length  of  the  waves  in  deep 
water,  it  is  possible,  by  counting  the  number  of  breakers  arriving 
at  the  shore  in  a  given  time,  to  determine  the  lengths  of  the 
waves  in  the  open  sea.  In  this  manner  it  has  been  established 
that  the  swell  in  the  open  sea  must  not  infrequently  have  lengths 
of  from  1000  to  2000  feet,  and  occasionally  moreii".  Now  in 
deep-water  waves  a  great  wave  length  means  a  great  velocity, 
and  some  authorities  doubt  whether  short  storm  waves  will 
lengthen  to  form  the  longer  swells,  since  this  would  mean  that 
the  speed  of  the  waves  was  accelerated  after  the  wind  ceased  to 
act  upon  them.  Antoine^^^  however,  believes  that  just  such 
an  acceleration  does  occur.  Others  suppose  that  the  waves 
are  propagated  by  gravity  at  the  same  rate  of  speed  given 
them  by  the  wind,  or  even  that  their  velocity  suffers  a  slight 
diminution.  Cornish  concludes  that  the  longer  swells  are 
present  during  storms,  but  are  obscured  by  the  shorter  waves 
which  are  then  more  prominent"^.  We  shall  find  later  that 
the  longer  waves,  while  they  agitate  the  surface  less  than  storm 
waves,  agitate  the  deeper  waters  much  more,  and  have  an  im- 
portant effect  upon  the  shoreline. 

Wave  Velocity.  —  The  velocity  of  oscillatory  waves  is  a  mat- 
ter 'of  considerable  interest  in  various  connections.  We  have 
already  observed  that  the  wave  form  travels  at  a  speed  very 
much  greater  than  that  of  the  water  particles  themselves.  Thus, 
a  wave  400  feet  long  and  15  feet  high  will  have  a  velocity  of 
about  45  feet  per  second,  while  the  surface  water  particles  will 
move  round  in  their  orbits  at  a  speed  of  but  5|  feet  per  second. 
For  ocean  waves  of  large  size  the  wave  velocity  is  apt  to  be  six 
or  seven  times  as  great  as  the  orbital  velocity;  but  it  is  im- 
possible to  give  any  definite  rule  for  the  relations  of  these  two 
elements  of  wave  motion"^. 

We  can  correlate  the  velocity  of  wave  motion  with  wave 
length  more  precisely,  however,  for  in  deep  water  the  velocity  of 
the  wave  depends  on  its  length,  and  is  proportional  to  the  square 
root  of  its  length^i^  The  velocity  of  any  wave  whose  length  is 
known  may  be  calculated  approximately  by  very  simple  for- 
mulae. Thus,  the  velocity  in  miles  per  hour  is  equal  to  the 
square  root  of  2j  times  the  wave  length  measured  in  feet"°.  If 
it  is  desired  to  have  the  result  expressed  in  feet  per  second,  then 


30  WATER  WAVES 

the  velocity  in  feet  per  second  is  equal  to  2j  times  the  square 
root  of  the  length  in  feet^^*^.  According  to  the  first  formula  a 
wave  100  feet  long  will  have  a  velocity  of  15  miles  per  hour; 

for  

Velocity  =  V2i  X  length 

=  V21  X  100  =  V225 

=  15  miles  per  hour. 

Ac:ording  to  the  second  formula  the  same  wave  will  have  a 
velocity  of  22.5  feet  per  second;  for 


Velocity  =  2|  Vlength 

=  21  VlOO  =  21  X  10 
=  22.0  feet  per  second. 


If  we  reduce  the  15  miles  per  hour,  derived  from  the  first  formula, 
to  feet  per  second,  we  get  22  feet  per  second,  which  agrees  fairly 
well  with  the  result  obtained  by  the  second  formula.  We  may 
also  determme  the  approximate  velocity  of  a  wave  in  feet  per 
second  by  the  formula: 


Velocity  =  V5|  X  length 

which  becomes,  in  the  case  of  the  wave  described  above, 

Velocity  =  V5|  X  100 

=  22f  feet  per  second. 

As  Gaillard  has  pointed  out  in  commenting  on  the  above  formula, 
the  velocity  of  a  deep-water  wave  is  practically  the  same  as  that 
which  a  body  would  acquire  in  falling  through  a  distance  equal 
to  8  per  cent  of  the  wave  length"^. 

Because  of  the  relations  existing  between  wave  velocity,  wave 
length,  and  the  period  of  the  waves,  we  may  determine  the 
velocity  of  waves  in  other  ways.  Thus  the  velocity  of  the  wave 
in  knots  per  hour  is  roughly  equal  to  three  times  the  period  (in 
seconds) ^^^.  Or  if  we  transform  the  period  of  the  wave  into  the 
number  of  waves  per  minute  (wave-frequency),  then  the  velocity 
in  feet  per  minute  is  equal  to  the  wave  length  multiplied  by  the 
frequency.  The  velocity  in  miles  per  hour  may  be  found  by 
dividing  the  frequency  into  198^^^     Thus  if  the  wave  100  feet 


WAVE  VELOCITY  31 

in  length,  considered  above,  have  a  period  of  about  4|  seconds, 
then  the  velocity  in  knots  per  hour  is  roughly  13|,  for 
Velocity  =  3  X  period 

=  3  X  41 

=  13^  knots  per  hour. 

The  velocity  of  this  same  wave  in  feet  per  minute  will  be  1333; 
for  a  period  of  4|  seconds  means  a  frequency  of  13^  (60  ^  4|  = 
13^),  whence  we  have  the  following: 

Velocity  =  wavelength  X  frequency 

=  100  X  13| 

=  1333  feet  per  minute. 

This  agrees  roughly  with  the  velocities  previously  obtained, 
since  it  is  equivalent  to  a  speed  of  22.2  feet  per  second.  The 
velocity  as  determined  from  the  frequency  alone  is  14.85  miles 

per  hour;  for 

Velocity  =  198  -^  frequency 
=  198  H-  13| 
=  14.85  miles  per  hour. 

In  order  to  determine  the  velocity  of  a  set  of  waves  by  this  last 
method  it  is  only  necessary  to  count  the  number  of  times  per 
minute  some  floating  object  bobs  up  and  down  as  the  waves  pass 
under  it,  or  to  count  the  waves  as  they  rise  against  some  fixed 
object.  The  result  is  in  sufficiently  close  agreement  with  the 
velocity  of  15  miles  per  hour  determined  by  a  preceding  formula. 

The  periods  of  waves  are  more  easily  determined  than  are 
length  or  velocity,  for  which  reason  it  is  convenient  to  have 
in  tabular  form  the  lengths  and  velocities  of  deep-water  waves 
corresponding  to  given  periods.  The  table  on  the  following  page, 
taker  from  White's  "  Naval  Architecture  "i^",  covers  all  waves  of 
ordinar}^  size. 

Velocities  of  Shalloic-ivater  Waves.  — In  the  preceding  pages  we 
have  discussed  the  laws  controlling  the  velocities  of  deep-water 
waves.  Shallow-water  waves,  or  waves  whose  lengths  are  great 
compared  to  the  depth  of  the  water,  obey  different  laws.  It  is 
a  well-known  fact  that  such  waves  move  less  rapidly  than  deep- 
water  waves,  and  Gaillard  has  expressed  in  tabular  form  the 
relative  velocities  of  the  two  types,  assuming  equal  wave  lengths, 
but  varying  depths  of  water  for  the  shallow-water  wave,  with  a 
minimum  depth  equal  to  .05  of  the  wave  length'-^     The  velocities 


32  WATER  WAVES 

LENGTH  AND  VELOCITY  OF  DEEP-WATER  WAVES 

(After  While.) 


Speed  of  advance 

Period,  seconds 

Length,  feet 

Feet  per  second 

Knots  per  hour 

1 

5.12 

5.12 

3.03 

2 

20.49 

10.24 

6.07 

3 

46.11 

15.37 

9.10 

4 

81.97 

20.49 

12.14 

5 

•          128.08 

25.62 

15.17 

6 

184.44 

30.74 

18.21 

7 

251.04 

35.86 

21.24 

8 

327.89 

40.99 

24.28 

9 

414.99 

46.11 

27.31 

10 

512.33 

51.23 

30.35 

11 

619.92 

56.36 

33.38 

12 

737.76 

61.48 

36.42 

13 

865.84 

66.60 

39.45 

14 

1004.17 

71.73 

42.49 

15 

1152.74 

76.85 

45.52 

16 

1311.56 

81.97 

48,56 

of  shallow-water  waves  of  this  type  must  be  calculated  by  means 
of  a  formula  less  simple  than  those  given  for  deep-water  waves, 
since  the  formula  must  be  applicable  to  varying  depths  of  water 
Such  a  formula,  and  numerous  comparisons  of  the  observed 
velocities  of  shallow-water  waves  with  the  velocities  computed 
by  the  formula,  are  given  in  Gaillard's  treatise  on  "  Wave 
Action  "122.  When  the  wave  length  is  more  than  1000  times  the 
depth  of  the  water,  the  velocity  depends  wholly  upon  the  depth 
according  to  Airy,  and  is  proportional  to  the  square  root  of  the 
depth.  The  velocity  of  such  a  wave  is  the  same  as  the  velocity 
which  a  body  would  acquire  by  falling  through  a  distance  equal 
to  half  the  depth  of  the  water^^l  This  is  the  law  for  the  velocity 
of  the  wave  of  translation  as  determined  by  Russell^-^;  and  it 
should  l^e  noted  that  Airy  is  inclined  to  regard  the  wave  of 
translation  as  merely  a  variety  of  the  wave  of  oscillation^^s  n 
is  also  interesting  to  note  that  while  this  law  is  called  Airy's  law 
or  formula  by  some,  and  is  named  for  Russell  by  others,  it  was 
really  applied  by  Lagrange  to  water  waves  at  least  as  early  as 
1788^-^  and  is  therefore  better  known  as  the  Lagrange  Formula. 
The  law  does  not  hold  good  for  very  shallow  depths,  according 
to  Calignyi27.   j^qj.  jj^  moving  water,  according  to  Moller^^^. 


WAVES  OF  TRANSLATION  33 

The  waves  generated  in  the  ocean  by  earthquakes  and  sub- 
marine volcanic  explosions  have  lengths  which  are  great  in  com- 
parison to  the  depth  of  the  ocean,  and  must  therefore  obey  the 
laws  controlling  the  movements  of  shallow-water  waves.  If 
we  determine  the  velocity  of  such  a  wave,  therefore,  we  should 
be  able  to  secure  some  idea  of  the  depth  of  the  ocean  it  traverses. 
This  was  first  done  by  Bache,  who  estimated  the  mean  depth  of 
the  North  Pacific  Ocean  (4200  to  4500  meters)  from  the  veloc- 
ity of  a  wave  produced  by  the  Simoda  earthquake  in  1854;  and 
later  others  followed  his  example  in  the  cases  of  the  Iquique 
earthquake  and  the  Krakatoa  explosion^-^.  The  calculations  are 
necessarily  inaccurate  for  various  reasons,  but  are  nevertheless 
of  considerable  interest. 


WAVES   OF  TRANSLATION 

Thus  far  we  have  confined  our  attention  to  waves  of  oscil- 
lation, in  which  the  water  particles  move  forward  on  the  crest 
and  backward  in  the  trough.  There  is  another  type  of  wave 
which  is  also  of  great  interest  to  the  student  of  shorelines,  al- 
though its  importance  is  not  always  appreciated.  This  is  the 
"  wave  of  translation,"  in  which  the  water  particles  move  for- 
ward as  the  wave  passes,  but  do  not  exhibit  a  compensating 
backward  motion.  While  not  important  on  the  ope';'  sea,  this 
type  of  wave  is  extensively  developed  in  the  shallow  waters 
along  all  coasts,  the  waves  of  oscillation  generated  in  deep  water 
frequently  becoming  more  or  less  completely  transformed  into 
waves  of  translation  as  they  approach  the  shore. 

Fonn.  —  The  wave  of  translation  was  discovered  by  Russell, 
and  described  at  length  by  him  in  his  reports  to  the  British  Asso- 
ciation^^".  He  showed  that  when  a  volume  of  water  was  suddenly 
added  to  the  still  water  in  a  canal,  or  when  a  portion  of  the  canal 
water  was  displaced  by  suddenly  plunging  a  solid  body  into  it, 
or  when  the  canal  water  was  pushed  into  a  mound  by  the  shoving 
motion  of  a  boat  or  of  a  plate  held  vertically,  a  single  prominent 
wave  rolled  forward  over  the  canal  surface.  The  entire  form  of 
this  wave  rose  above  the  still-water  surface  of  the  canal,  and 
included  no  trough  such  as  constitutes  part  of  the  wave  of  oscil- 
lation. A  careful  examination  of  the  newly  discovered  wave 
showed  that  it  differed  widely  from  oscillatory  waves  in  other 


34  WATER  WAVES 

respects,  and  that  the  motion  of  its  water  particles  made  the 
name  "  wave  of  translation "  appropriate.  Let  us  consider 
briefly  the  essential  characters  of  this  wave,  turning  our  attention 
first  to  the  nature  of  the  movements  executed  by  the  water 
particles. 

Motion.  —  Immediately  before  and  immediately  after  the  pass- 
ing of  a  wave  of  translation,  the  surface  of  the  water  and  the 
water  particles  in  depth  may  be  quite  still.  During  the  passage 
of  the  wave  the  surface  water  particles  rise  and  move  forward, 
descending  again  to  the  original  level,  but  to  an  advanced  po- 
sition horizontally,  where  they  come  to  rest.  Thus  in  Figure  6 
the  particle  a  rises,  moves  forward  and  descends  to  the  position  h. 
Water  particles  below  the  surface  move  forward  the  same  dis- 
tance, but  their  vertical  rise  diminishes  with  increase  in  depth. 


Fig.  6.  —  Diagram  showing  movement  of  water  particles  in  a  wave  of  trans- 
lation.    (After  Russell.) 

The  paths  described  by  the  water  particles  are  semi-ellipses 
which  have  their  major  axes  horizontal  and  equal,  and  their 
minor  axes  progressively  shorter  as  the  distance  below  the  sur- 
face increases,  until  on  the  bottom  the  path  becomes  a  straight 
line'^^  It  will  be  seen  from  the  figure  that  water  particles  ver- 
tically above  each  other,  as  aceg,  come  to  rest  in  the  same 
relative  position  farther  on  at  bdfh.  There  is  thus  a  real  and 
permanent  forward  translation  of  the  water  itself  through  a  short 
distance,  in  addition  to  the  forward  transmission  of  the  wave 
form  through  a  very  great  distance.  The  space  through  Vv^hich 
the  water  particles  are  moved  forward  is  just  large  enough  to 
contain  the  volume  of  water  in  the  wave  above  still-water  level. 
Manifestly  there  are  several  points  connected  with  the  motion 
of  the  water  particles  in  waves  of  translation  which  will  prove  of 
importance  when  we  come  to  discuss  the  effect  of  waves  upon 
shores.  The  fact  that  the  water  particles  advance,  but  do  not 
have  a  compensating  backward  motion  should  result  in  effective 
transportation  of  sand  and  gravel  on  shallow  sea-bottoms  in  the 
direction  of  wave  propagation,  unless  other  forces  prevent.     It 


WAVES  OF  TRANSLATION  35 

is  likewise  worthy  of  note  that  in  waves  of  translation  the  bottom 
particles  move  forward  just  as  far  as  do  the  surface  particles, 
whereas  we  have  already  seen  that  in  oscillatory  waves  the  move- 
ment of  the  water  particles  dies  out  rapidly  below  the  surface. 
Evidently  waves  of  translation  may  profoundly  affect  the  bottom 
to  great  depths,  although  this  conclusion  is  subject  to  the  quali- 
fication, subsequently  to  be  discussed,  that  waves  of  translation 
traversing  water  bodies  of  great  depth  as  compared  to  the  size 
of  the  waves,  tend  to  be  transformed  into  waves  of  oscillation. 
We  shall  see  later  that  only  one-half  of  the  energy  of  an  oscil- 
latory wave  is  transmitted  forward  with  the  wave  form,  whereas 
the  total  energy  of  a  wave  of  translation  is  thus  transmitted. 

Wave  Length.  —  The  length  of  the  typical  wave  of  translation 
is  measured  from  the  point  where  it  begins  to  rise  from  the  still- 
water  level  in  front  to  the  point  where  the  back  slope  of  the  wave 
again  merges  with  still- water  level.  These  points  are  not  easily 
determined  with  accuracy,  but  according  to  Russell  the  wave 
length  thus  measured  on  artificial  waves  is  "equal  to  about  six 
times  the  depth  of  the  fluid  below  the  plane  of  repose."  The 
height  of  the  wave  above  the  still-water  surface  may  be  equal  to 
the  depth  of  the  fluid  in  repose,  but  cannot  exceed  this  measure, 
as  the  wave  breaks  whenever  the  height  becomes  equal  to  the 
depth^^-.  Actual  measurements  of  wave  heights  and  lengths  in 
nature  are  usually  made  upon  the  open  sea  or  in  other  localities 
favorable  to  the  formation  of  waves  of  oscillation ;  and  while  it  is 
possible  that  some  of  the  figures  previously  given  are  really  those 
for  waves  of  translation,  no  distinction  is  usually  made  by  the 
observer,  and  we  lack  proper  data  for  the  range  in  size  of  natural 
waves  of  translation. 

Velocity.  —  The  velocity  of  the  wave  of  translation  depends  upon 
the  depth  of  the  water  measured  from  the  crest  of  the  wave,  and 
varies  as  the  square  root  of  the  depth.  Otherwise  expressed,  the 
velocity  of  the  wave  is  the  same  as  the  velocity  which  a  heavy 
body  will  acquire  by  falling  freely  through  a  distance  equal  to 
half  the  depth  of  the  fluid  below  the  wave  crest^'^l  In  the  deep 
ocean  such  waves  should  have  very  high  velocities,  and  doubtless 
many  of  the  earthquake  waves  which  traverse  the  ocean  with 
velocities  from  a  few  hundred  miles  to  nearly  a  thousand  miles 
an  hour^^*  are  true  waves  of  translation;  while  the  tidal  wave 
which  has  a  velocity  of  from  480  to  660  miles  an  hour  in  depths  of 


36  WATER  WAVES 

between  12,000  and  20,000  feet^^^  is  a  compound  wave  having 
some  of  the  characteristics  of  the  wave  of  translation.  In  very 
shallow  water  the  velocities  of  waves  of  translation  must  of 
necessity  be  very  low. 

Com'plexitieH  of  ^Yaves  of  Translation .  —  According  to  Caligny  the 
waves  of  translation  are  not  alwaj'S  so  simple  in  character  as 
supposed  by  Russell.  The  French  engineer  made  a  series  of  ex- 
periments which  led  him  to  conclude  that  there  are  waves  of 
translation  in  which  the  water  particles  describe  closed  orbits; 
that  these  orbits  may  approximate  vertical  ellipses,  but  that  the 
backward  movement  of  the  water  particles  ma}'  slightly  exceed 
the  forward  movement,  causing  material  on  the  bottom  to  be 
transported  in  a  direction  opposite  to  that  of  wave  propaga- 
tion; and  that  a  solitarj"  wave  of  translation  may  pass  through 
a  series  of  oscillatory  waves,  complicating  their  form  and  causing 
them  to  break.  He  also  pointed  out  that  it  is  possible  to  have  a 
succession  of  solitarv  waves  of  translation  which  will  resemble 
ordinary  waves  of  oscillation,  the  spaces  between  the  waves 
resembling  troughs  so  closel}'-  as  to  mislead  the  observer^^^. 

The  investigation  of  waves  of  translation  in  nature  is  further 
complicated  b}^  the  fact  that  notwithstanding  Hunt's  arguments 
to  the  contrary^^^,  normal  waves  of  oscillation  appear  to  be  grad- 
ually transformed  into  waves  of  translation  when  they  enter 
water  which  slowly  decreases  in  depth,  and  hence  all  intermediate 
phases  between  the  two  types  of  waves  may  be  encountered. 
When  the  tops  of  Ijreakers  fall  forward,  the  volume  of  vrater  thus 
added  to  the  water  surface  in  front  produces  waves  of  translation 
which  run  on  shore,  mingling  with  waves  of  oscillation.  If 
waves  of  translation  encounter  a  cliff  or  steep  shore,  they  may 
be  reflected,  the  direction  of  the  transport  of  water  particles  in 
the  reflected  v/ave  being  seaward.  For  these  and  other  reasons 
which  will  presentl}^  pppear,  it  may  be  practicall}^  impossible 
to  determine  the  nature  of  the  water  movements  which  are 
affecting  the  distribution  of  sand  and  gravel  along  a  shelving 
coast. 

Such  complications  should  not,  however,  make  us  lose  sight 
of  the  importance  of  waves  of  translation  as  agents  of  shoreline 
changes.  Under  favorable  conditions  the  operation  of  these 
waves  may  easily  be  observed.  Thus,  when  large  swells  en- 
counter the  seaward   margin  of  a  submarine  terrace  (Fig.  7), 


WA\ES   OF  TRAXSLATIOX 


37 


they  break  and  form  smaller  waves  of  trans- 
lation which,  on  a  calm  day,  may  cross  the 
shallow  water  to  the  shore  without  deforma- 
tion until  they  break  as  a  secondary  surf  on 
the  beach.  The  level  water  surface  between 
any  two  waves  of  translation  may  be  seen  to 
differ  distinctly  from  the  true  trough  of  the 
oscillator}^  wave  in  deeper  water.  Russell  ob- 
served a  striking  example  of  waves  of  transla- 
tion, formed  in  the  manner  above  described, 
on  the  shore  of  Dublin  Bay,  and  thus  de- 
scribes the  phenomena: 

"'  One  of  the  common  sea  waves,  being  of  the 
second  order  (waves  of  oscillation),  approaches 
the  shore,  consisting  as  usual  of  a  negative  or 
hollow  part,  and  of  a  positive  part  elevated 
above  the  level;  ....  At  length  the  wave 
breaks,  and  the  positive  part  of  the  wave  falls 
forward  into  the  negative  part,  filling  up  the 
hollow  ....  After  a  wave  has  first  been  made 
to  break  on  the  shore,  it  does  not  cease  to 
travel,  but  if  the  slope  be  gentle,  and  the 
beach  shallow  and  very  extended  (as  it  some- 
times is  for  a  mile  inwards  from  the  breaking 
point,  if  the  waves  be  large),  the  whole  inner 
portion  of  the  beach  is  covered  with  positive 
waves  of  the  first  order  (waves  of  translation), 
from  among  which  all  waves  of  the  second 
order  have  disappeared.  This  accounts  for  the 
phenomenon  of  breakers  transporting  shingle 
and  wreck,  and  other  substances  shorewards 
after  a  certain  point."  Then  referring  more 
particularly  to  the  conditions  at  Dublin  Bay, 
he  says  that  the  "  waves  coming  in  from  the 
deep  sea  are  first  broken  when  they  approach 
the  shallow  beach  in  the  usual  way;  they  give 
off  residuary  waves,  which  are  positive  (waves 
of  translation);  these  are  wide  asunder  from 
each  other,  are  wholly  positive  (i.e.,  above 
still- wa^er  level),  and  the  spaces  between  them, 


38  WATER  WAVES 

several  times  greater  than  the  ampHtude  of  the  wave,  are  per- 
fectly flat;  and  in  this  condition  they  extend  over  wide  areas 
and  travel  to  great  distances  "^^^. 


EARTHQUAKE  AND   EXPLOSION   WAVES 

In  investigations  of  shoreline  changes  the  student  may  have 
occasion  to  refer  to  another  class  of  waves  whi(;h  are  occasionally 
developed  upon  the  ocean,  and  which  are  improperly  called  "  tidal 
waves."  These  are  the  waves  of  enormous  size  and  destructive 
energy  produced  by  su])marine  earthquakes  and  volcanic  explo- 
sions, and  for^which  Ho1jI)s''''-*  has  suggested  adopting  the  Japanese 
name  "tsunamis."  They  occur  at  such  rare  intervals,  and  oper- 
ate for  such  a  brief  p(n-iod,  that  they  are  probably  not  of  great 
importance  in  modeling  the  forms  of  the  shore.  But  inasmuch 
as  they  temporarily  raise  the  upper  limit  of  salt  water  far  above 
its  normal  position,  and  leave  behind  them  records  which  may 
be  mistaken  as  evidences  of  a  former  higher  stand  of  th(;  mean 
sealevel,  it  is  important  that  we  become  familiar  with  the  work 
of  these  waves. 

Nature  and  Origin  of  Wave  Motion.  —  A  submarine  earthquake 
may  produce  several  types  of  waves.  There  are  first  the  short  and 
quick  oscillations  which  travel  toward  the  surface  with  the  velocity 
of  sound  in  water,  and  which  ai'e  felt  by  overlying  vessels  as  a  sharp 
and  violent  shock,  often  causing  the  sailors  to  believe  that  the  ves- 
sel has  struck  a  reef.  Old  charts  contain  many  isolated  shallows 
and  reefs  reported  by  vessels  which  had  really  experienced  earth- 
quake shocks  in  deep  water.  Occasionally  such  shocks  are  severe 
enough  to  hurl  the  ship  out  of  water,  to  break  off  its  inasts,  or  even 
to  destroy  the  vessel  (;ntir(!ly'^".  These  oscillations  are  not  of  the 
type  which  produce  promincnit  surface  waves,  however.  Other 
groups  of  waves  are  prc^duced  by  the  dislocation  of  the  sea-bot- 
tom. While  the  mechanism  of  these  dislocation  waves  is  not 
well  understood,  it  is  probable  that  the  uplifting  of  a  portion  of 
the  sea-bottom  raises  a  mound  of  water  above  the  general  sur- 
face of  the  sea,  and  that  the  settling  back  of  this  water  generates 
a  great  wave  of  translation  which  traverses  the  ocean  with  high 
velocity.  Sometimes  several  such  waves  are  produced,  possibly 
by  the  disintegration  of  a  former  single  wave  of  translation 
after  the  manner  described  by  Russell  for  some  of  his  experi- 


EARTHQUAIvE  AND  EXPLOSION  WAVES  39 

ments'^^  The  sudden  settling  of  a  submarine  crust  block  may 
generate  a  negative  wave  of  translation.  On  the  other  hand,  the 
behavior  of  many  earthquake  waves  upon  reaching  the  coast 
suggests  that  they  partake  of  the  characters  of  oscillatory  waves, 
the  water  particles  moving  backward  in  a  sort  of  great  trough 
toward  the  oncoming  wave  crest.  According  to  Reid  the  waves 
caused  by  the  same  earthquake  fii'st  appear  as  a  depression  of 
the  water  at  some  ports,  and  as  an  elevation  at  others;  a  fact 
which  he  attempts  to  explain  on  the  theory  that  the  down- 
dropped  block  generates  a  negative  wave  and  the  upraised  block 
a  positive  wave"-.  It  is  possible  that  the  phenomena  in  question 
may  be  explained  as  a  result  of  the  different  velocities  with 
which  positive  and  negative  waves  are  propagated,  both  having 
resulted  from  the  return  of  a  mass  of  water  raised  above  the 
general  level,  in  some  such  manner  as  that  described  by  Russell 
for  his  "  residuary  negative  waves  ""I  Our  knowledge  of  earth- 
quake waves  is  still  too  meager,  however,  to  enable  us  to  spi^ak 
with  assurance  on  this  and  other  questions  concerning  their 
behavior.  An  experimental  study  of  their  mode  of  piopa- 
gation  will  be  found  in  the  Weber  brothers'  "  Wellenlehre  ""S 
a  full  resume  of  our  present  knowledge  of  the  subject  in  Kriim- 
mel's  "  Ozeanographie  ""^  and  a  good  brief  statement  in  Thou- 
let's  "  Oceanographie  Dynamique  """. 

In  submarine  volcanic  explosions  there  is  also  produced  a 
sharp  and  powerful  shock,  corresponding  exactly  to  the  first 
mentioned  effect  of  earthquakes.  At  this  time  small  jets  of 
water  may  be  shot  into  the  air;  but  there  soon  follows  a  doming 
or  up-swelling  of  the  ocean  surface,  and  finally  the  whole  mass 
of  up-raised  water  may  be  hurled  into  the  air  by  the  escaping 
gases.  The  doming  of  the  water,  ihe  push  exerted  by  the  gases, 
and  the  back-falling  mass  of  water,  all  tend  to  produce  waves, 
some  of  which  are  waves  of  translation,  and  some  probal^ly  oscil- 
latory or  compound  waves"''.  Explosion  waves  and  dislocation 
waves  cannot  be  distinguished,  and  the  origin  of  many  of  these 
waves,  often  designated  collectively  as  "  earthquake  waves," 
remains  in  doubt.  According  to  Kriimmel,  Rudolph  supposed 
that  the  great  wave  which  overwhelmed  Lisbon  following  the 
earthquake  of  1755  was  due  to  a  volcanic  explosion  near  the 
Portuguese  coast"^  Most  authorities  agree  that  the  waves  which 
followed  the  eruption  of  Krakatoa  in  1883  were  due  directly  to 


40  WATER  WAVES 

the  force  of  the  explosion  itself,  but  some  have  argued  that  they 
resulted  from  the  masses  of  rock  falling  back  into  the  water"^ 

On  the  open  sea  the  heights  of  earthquake  and  explosion  waves 
quickly  diminish,  and  since  the  lengths  are  very  great,  they  soon 
become  so  low  and  flat  as  to  be  unnoticed  by  vessels.  But 
when  they  enter  shallow  water  they  behave  like  other  waves, 
the  height  increasing  until  the  wave  form  breaks  to  produce  a 
gigantic  surf.  The  velocity  of  these  waves  is  very  great,  as  they 
may  travel  a  distance  of  9000  or  10,000  miles  in  24  hours,  and 
one  instance  is  recorded  in  which  a  velocity  of  900  miles  an  hour 
was  attained^^".  Their  periods  range  from  15  minutes  to  one  or 
two  hours,  and  by  assuming  them  to  be  the  periods  of  free  waves 
in  deep  water  it  has  been  calculated  that  the  lengths  of  earth- 
quake and  dislocation  waves  vary  from  100  miles  to  600  miles  or 
more^^^ 

Recorded  Heights.  —  As  students  of  shoreline  phenomena  we  are 
more  interested  in  the  height  attained  by  this  class  of  waves  when 
they  reach  the  coast.  We  can  better  appreciate  the  truly  surpris- 
ing elevations  at  which  they  may  leave  evidences  of  their  former 
presence  if  we  review  some  of  the  actual  cases  of  which  we  have 
authentic  records.  In  the  years  358  and  365  A.D.,  the  eastern 
shore  of  the  Mediterranean  was  visited  by  great  waves  which  passed 
over  islands  and  low  shores,  sweeping  away  buildings  and  thou- 
sands of  people.  Boats  were  left  on  the  roofs  of  houses  in  Alexan- 
dria, and  others  were  stranded  nearly  a  mile  inland  near  Modhoni 
where  they  were  later  found  slowly  decaying^'".  Following  the 
Lisbon  earthquake  in  1755  a  wave  variously  estimated  as  from 
40  to  60  feet  high  broke  on  the  coast  at  Cadiz.  The  great  earth- 
quake at  Lima  in  1724  was  followed  by  a  wave  said  to  have  been 
80  feet  high  and  which  carried  four  vessels  far  inland.  In 
August,  1868,  an  earthquake  on  the  coast  of  Peru  resulted  in 
large  waves,  one  of  which  submerged  the  mainland  55  feet  above 
high-water  mark.  A  United  States  war  vessel  was  carried  a 
quarter  of  a  mile  inland  at  Arica,  where  it  remained  until  an- 
other great  wave  carried  it  still  farther  inland  in  1877.  This 
last  was  the  wave  caused  l^y  the  Iquique  earthquake,  and  it  is 
said  to  have  varied  in  height  from  20  to  80  feet.  An  earthquake 
on  the  island  of  Hondo,  Japan,  in  1854  was  accompanied  by  a 
wave  which  rose  30  feet  above  the  usual  level  of  the  water.  In 
1896  another  disturbance  on  the  same  coast  generated  three 


TIDAL  WAVES  41 

waves,  the  largest  of  which  was  50  feet  high  on  the  shore.  Ships 
were  torn  from  their  anchorage,  and  one  two-masted  schooner 
was  washed  nearly  a  third  of  a  mile  inland.  The  Messma  earth- 
quake of  December  28,  1908,  produced  waves  winch  rose  nearly 
30  feet  high  on  some  of  the  adjacent  coasts.  Following  the 
eruption  of  Krakatoa  in  1883  waves  of  enormous  height  wrought 
destruction  over  great  distances.  On  the  southern  end  of 
Sumatra  one  wave  was  over  70  feet  high,  and  carried  a  gunlwat 
two  miles  inland  where  it  was  left  30  feet  above  sealevel.  In 
Katimbong  the  wave  rose  80  feet,  and  on  the  shallow  shore  of 
Merak,  on  the  Java  coast,  reached  the  enormous  height  of  115 
to  135  feeti^^ 

It  is  evident  that  such  great  waves  must  leave  many  records 
of  their  presence  far  above  the  normal  level  of  the  sea.  Not 
only  large  vessels  and  smaller  l)oats,  which  readily  attract  the 
popular  attention,  but  fish  and  other  forms  of  marine  life  are 
left  stranded  far  inland  and  high  above  the  reach  of  the  highest 
tides  or  greatest  storm  waves.  The  bones  of  whales,  and  well- 
preserved  marine  sheik  occasionally  found  high  above  the  sea, 
must  not  too  readily  be  accepted  as  proof  of  a  very  recent  uplift 
of  the  land.  Successive  earthquake  waves  in  a  given  ocean 
may  deluge  the  coasts  of  all  the  surrounding  continents;  and 
we  must  therefore  expect  to  find  driftwood,  shells,  and  bones 
of  fish  well  above  sealevel  at  occasional  points  on  almost  any 
shore. 

TIDAL  WAVES 

The  great  periodic  motion  of  the  sea  known  as  the  tide  com- 
bines some  of  the  features  of  oscillatory  waves  with  others  be- 
longing to  waves  of  translation.  It  has  been  described  by 
Russell  as  a  ''  compound  wave  of  the  first  order  "  (wave  of 
translation)  having  more  of  the  characteristics  of  waves  of  this 
order  than  of  oscillatory  waves^^*.  There  is  no  necessity,  however 
of  our  entering  into  a  discussion  of  the  origin  and  character  of 
the  tidal  wave,  since  the  only  elements  of  its  motion  of  vital  in- 
terest to  the  student  of  shore  forms  are  the  currents  it  produces, 
and  the  height  to  which  it  rises;  both  of  which  points  are  con- 
sidered in  another  part  of  this  volume. 

Wheeler  has  expressed  the  belief  that  the  rising  and  falling 
of  the  tide  is  accompanied  by  the  production  of  "  tidal  wavelets  " 


42  WATER  WAVES 

which  are  not  the  result  of  wind  action,  but  are  in  some  way 
genetically  related  to  the  tide  itself  ^^^  The  explanation  of  their 
origin  which  he  gives  is  not  wholly  satisfactory,  and  his  theory 
seems  to  be  based  upon  his  observation  that  waves  from  6  to  24 
inches  in  height  break  upon  the  beach  at  the  rate  of  ten  to  twenty 
a  minute  "  when  there  is  an  entire  absence  of  wind  or  other  dis- 
turl^ing  cause."  In  the  absence  of  sufficient  evidence  to  connect 
such  wavelets  with  the  tides,  we  may  perhaps  more  safely  regard 
them  as  due  to  the  action  of  gentle  breezes  and  occasional  gusts 
of  wind,  possibly  some  distance  away,  which  even  on  the  calmest 
day  never  permit  the  ocean  surface  to  become  absolutely  quies- 
cent. Haupt^^'^  states  that  the  flood  tide  produces  waves  which 
break  obliquely  on  the  beach,  and  speaks  of  "  the  angle  at  which 
the  flood  breaks  upon  the  shore."  But  since  he  also  speaks  of 
these  supposed  tidal  waves  as  "  breakers  racing  along  the  shore," 
and  quotes  Mitchell's  description  of  the  manner  in  which  the 
"  larger  class  of  swell  or  rollers  "  strike  the  shore  as  an  example 
of  tidal  wave  activity,  it  would  appear  that  Haupt  has  mistaken 
the  ground-swell  of  distant  storms  for  tidal  waves.  A  similar 
misapprehension  may  have  been  responsible  for  Marsh's  curious 
idea  that  "  on  most  coasts  the  supply  of  sand  for  the  formation 
of  dunes  is  derived  from  tidal  waves,"  since  "  the  momentum 
acquired  by  the  heavy  particles  in  rolling  in  with  the  water 
tends  to  carry  them  even  beyond  the  flow  of  the  waves  "i^^. 


STANDING   WAVES;    SEICHES 

Under  certain  conditions  there  may  exist  oscillations  of  the 
water  known  as  standing  waves,  in  which  the  water  particles 
do  not  describe  closed  orbits,  but  return  through  the  same  paths 
by  which  they  advance  (Fig.  8).  The  surface  water  moves  up- 
ward in  all  of  the  crest,  and  downward  in  all  of  the  trough, 
and  the  vertical  movement  of  the  particles  is  at  a  maximum 
under  the  crest  where  the  horizontal  movement  is  niP^'^.  Hori- 
zontal movement  is  at  a  maximum  under  the  nodal  lines  (Fig.  8). 
An  example  of  the  standing  wave  is  the  seiche,  typically  developed 
in  inland  lakes,  and  extensively  studied  in  Lake  Geneva  by 
Forel.  This  movement  consists  of  a  periodic  rise  and  fall  of 
the  water  surface  which  is  initiated  by  winds  piling  up  the  water 
at  one  end  of  the  lake,  by  sudden  variations  in  atmospheric 


STANDING  WAVES;    SEICHES  43 

pressure,  by  earthquakes,  by  landslides,  or  by  some  other  dis- 
turbance; and  which  continues  for  some  time  with  gradually 
diminishing  intensity.  Each  body  of  water  has  its  own  period, 
appropriate  to  its  dimensions,  and  the  extent  to  which  the  water 
rises  and  falls  depends  on  the  dimensions  of  the  water  body  and 
the  nature  of  the  disturbing  force.  The  principal  seiche  on 
Lake  Geneva  has  an  amplitude  of  from  8  centimeters  to  2  meters^^^ 


Fig.  8.  —  Diagram  to  illustrate  the  movement  of  water  particles  in  standing 
waves,  such  as  the  seiche. 

Seiches  also  occur  along  the  coasts  of  the  ocean,  especially  in 
bays  and  straits.  Examples  of  these  and  other  types  of  seiches 
are  described  in  Harris's  "  Manual  of  Tides  "i*"',  and  Thoulet's 
"  Oceanographie  Dynamique  "^'^K  According  to  Dawson^''-  a 
seiche  at  Yarmouth,  Nova  Scotia,  had  an  amplitude  of  from  128 
to  143  centimeters,  or  a  maximum  change  of  level  of  nearly  5 
feet.  As  a  rule,  however,  most  seiches  have  an  amplitude  of  a 
few  inches  only.  The  period  varies  from  a  few  minutes  in  small 
water  bodies  to  many  hours  in  large  ones,  and  the  velocity  of 
the  water  particles  participating  in  the  oscillation  is  not  great. 
Indeed,  the  direct  effect  of  seiches  upon  shoreline  processes  is 
probably  almost  negligible.  In  the  rare  cases  where  the  am- 
plitude is  great  the  effect  of  seiches  is  temporarily  to  raise  the 
zone  of  ordinary  wave  activity  to  an  appreciable  extent;  and 
occasionall}"  the  rising  and  falling  of  the  water  will  cause  currents 
of  some  importance  through  narrow  straits  or  inlets;  but  these 
are  exceptional  cases  and  do  not  justify  us  in  devoting  further 
space  to  the  subject  of  seiches  in  this  connection.  A  good  ac- 
count of  this  type  of  wave  motion,  with  a  short  bibhography, 
will  be  found  in  the  work  by  Harris  already  referred  to,  while 
Darwin's  volume  on  "  Tides  and  Kindred  Phenomena  "  gives 
a  description  of  Forel's  important  researches  and  a  list  of  his 
classic  papers^^^. 


44 


WATER  WAVES 


BOUNDARY  WAVES 

Where  a  layer  of  lighter  surface  water  overlies  a  heavier 
water  stratum,  any  sudden  wind  which  creates  or  accelerates 
movement  of  the  surface  water  will  cause  a  rise  of  the  under- 
lying heavier  water  at  the  point  affected,  and  a  corresponding 
depression  in  the 
heavier  water  farther 
forward.  The  de- 
velopment of  such 
*'  boundary  waves  " 
at  the  plane  of  con- 
tact of  two  hquids  Fig.  9 
having  different  den- 
sities can  readily  be 


Boundary  wave  formed  by  local  air 
current  over  liquids  of  different  densities. 
(After  Sandstrom.) 


demonstrated  by  repeating  Sandstrom's  experiment,  in  which  one 
of  the  layers  was  colored  in  order  to  distinguish  it  from  the  other, 
and  a  local  air  current  was  artificially  generated^''-*.     (Fig.  9.) 

When  fresh  water  from  some  large  river  flows  out  over  the 
heavier  salt  water  of  the  sea,  conditions  favoring  the  formation 
of  boundary  waves  exist.  Such  waves  move  very  slowly,  their 
velocities  depending  upon  the  difference  in  density  of  the  two 

water  layers  and  in- 
creasing with  the 
square  root  of  this 
differenced''^  If  the 
generating  wind 
cease,  the  boundary 
waves  advance  to 
the  margins  of  the 
containing  b  a  s  i  n, 
where  they  are  par- 
tially destroyed  and 
partly  reflected  back 
beneath  the  surface. 
In  their  progress  they  give  rise  to  surface  waves  of  the  same 
length,  but  much  smaller  height,  the  crests  of  the  surface  waves 
being  directly  above  the  troughs  of  the  boundary  waves^^^.  Since 
boundary  waves  have  a  low  velocity,  and  the  water  particles 
involved  move  still  more  slowly,  it  may  be  doubted  whether  they 


Fig.  10.  —  Diagram  showing  movement  of  water 
particles  in  overlying  fresh  water  (white)  and 
underl3dng  salt  water  (shaded)  during  the 
passage  of  boundary  waves  from  left  to  right 
(long  arrow).      (After  V.  W.  Ekman.) 


RESUME  45 

are  of  importance  in  shore  processes.  A  good  brief  summary  of 
the  character  of  these  waves  is  pubhshed  by  Helland-Hansen 
and  Nansen  in  their  report  on  the  Norwegian  Sea'*^^,  while  the 
mathematical  theory  applicable  to  them  has  been  developed  by 

Stokesi««. 

RESUME 

In  the  foregoing  pages  we  have  gained  some  idea  of  the  nature 
of  that  force  which  is  the  most  important  agent  in  the  modeling 
of  shore  forms.  We  have  considered  the  form  and  charac- 
teristics of  waves  on  the  deep  sea  in  order  that  we  might  the 
better  appreciate  the  changes  which  they  undergo  as  they 
approach  the  coast  and  begin  their  geological  work.  The  mo- 
tion of  the  water  particles  in  different  types  of  waves;  the  nature 
of  wave  motion  in  deep  water  and  over  shallow  sea-bottoms; 
the  origin  of  storm  waves,  swells,  and  surf;  the  magnitude  of 
waves  and  the  conditions  which  govern  their  size;  and  the 
velocity  of  wave  advance  in  both  deep  and  shallow  water,  have 
in  turn  received  our  attention.  With  these  points  in  mind  we 
are  prepared  to  enquire  into  the  energy  expended  by  waves 
upon  the  shore,  and  the  work  thereby  accomplished. 

In  spite  of  the  apparently  hopeless  chaos  presented  by  the 
surface  of  a  stormy  sea,  we  know  that  the  waves  are  controlled 
by  definite  natural  laws,  and  that  the  different  elements  of  form 
and  motion  are  in  systematic  relation  to  one  another.  So  per- 
fect is  this  relationship  that  one  may  stand  upon  the  beach  and 
time  the  breakers  as  they  dash  themselves  to  pieces  at  his  feet, 
and  learn  thereby  the  length  and  velocity  which  these  same 
waves  had,  hours  ago,  far  away  upon  the  deep  sea.  On  the 
other  hand,  we  know  that  not  all  the  laws  which  control  the 
behavior  of  waves  have  been  discovered;  and  we  have  seen 
that  where  different  types  of  waves  act  simultaneously  upon 
the  same  water  body  it  may  be  difficult  or  even  impossible  to 
analyze  the  resultant  movements  of  the  water.  We  are  there- 
fore prepared  to  find  that  through  the  work  of  waves  upon  a 
coast  the  shoreline  is  changed  according  to  definite  natural 
laws  which  are  in  part,  at  least,  discoverable.  But  we  shall  not 
be  surprised  if,  in  the  present  state  of  our  knowledge  of  waves, 
we  find  it  impossible  to  explain  all  of  the  changes  which  take 
place  upon  a  shore  under  their  influence. 


46  WATER  WAVES 

REFERENCES 

1.  Kelvin,  Lord  (Sir  William  Thompson).     Popular  Lectures  and  Ad- 

dresses.    Ill,  Navigation,  511  pp.,  London,  1891. 

2.  Fleming,  J.  A.     Waves  and  Ripples  in  Water,  Air,  and  ^Ether.     299 

pp.,  London,  1902. 

3.  Cornish,  Vaughan.     Waves   of   the   Sea  and  Other  Water  Waves. 

374  pp.,  Chicago,  1911. 

4.  Fleming,  J.  A.     Waves  and  Ripples  in  Water,  Air,  and  ^ther.     299  pp., 

London,  1902. 

5.  Russell,  J.  Scott.     Report  on  Waves,  made  to  the  Meetings  in  1842 

and  1843. 
Report  of  the  British  Association.     XIV,  375-381,  1844  (1845). 

6.  Russell,  J.  Scott.     Report  of  the  Committee  on  Waves,  appointed  by 

the  British  Association  at  Bristol  in  1836,  etc. 

Report  of  the  British  Association.    VII,  417-496,  1837  (1838). 
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and  1843. 

Report  of  the  British  Association.     XIV,  311-390,  1844  (1845). 

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374  pp.,  Chicago,  1911. 

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345*-350,*  1848. 

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and  1843. 

Report  of  the  British  Association.     XIV,  337,  1844  (1845). 

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formen  des  Meeres,  p.  12,  Stuttgart,  1911. 

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Maritimes.     188  pp.,  Paris,  1831. 

14.  Russell,  J.  Scott.     Report  of  the  Committee  on  Waves,  appointed  by 

the  British  Association  at  Bristol  in  1836,  etc. 

Report  of  the  British  Association.     VII,  417-496,  1837  (1838). 
Russell,  J.  Scott.     Report  on  Waves,  made  to  the  Meetings  in  1842 
and  1843. 
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15.  Russell,  J.  Scott.     The  Modern  System  of  Naval  Architecture.     3 

Vols.,  London,  1865. 

16.  Bazin,   Henri.     Recherches   Experimentales  sur  la    Propagation    des 

Ondes.      Mem.  de  I'Acad.  des  Sciences  de  I'lnst.  de  France  XIX, 
495-644,  1865. 

17.  Airy,   G.   B.     On  Tides  and  Waves.      Encyclopaedia   Metropolitana. 

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REFERENCES  47 

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21.  Bertin,  Emile.     Etude  sur  la  Houle  et  le  Roulis.     Memoires  de  la 

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Societe  Nationale  des  Sciences  Naturelles  de  Cherbourg.  XVI, 
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25.  Cialdi,  Alesbandro.     Sul  Moto  Ondoso  del  Mare  e  su  le  Correnti  di 

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26.  Caligny,  a.  de.     Oscillations  de  I'Eau.     964  pp.,  Paris,  1883. 

27.  Stevenson,    Thomas.     The    Design    and    Construction    of    Harbours. 

3rd  Edition.     355  pp.,  Edinburgh,  1886. 

28.  Fleming,  J.  A.     Waves  and  Ripples  in  Water,  Air,  and  JEther.     299 

pp.,  London,  1902. 

29.  Wheeler,  W.  H.     A  Practical  Manual  of  Tides  and  Waves.     201  pp., 

London,  1906. 

30.  Wheeler,  W.  H.     The  Sea  Coast:    Destruction:    Littoral  Drift:    Pro- 

tection.    361  pp.,  London,  1902. 

31.  Cornish,    Vaughan.     Waves   of   the   Sea   and   Other   Water   Waves. 

374  pp.,  Chicago,  1911. 

32.  KRtiMMEL,    Otto.     Handbuch    der    Ozeanographie.     II.    Die    Bewe- 

gungsformen  des  Meeres.     766  pp.,  Stuttgart,  1911. 

33.  Gaillard,   D.  D.,     Wave  Action  in  Relation  to  Engineering  Struc- 

tures. Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31. 
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34.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  731  pp., 

London,  1900. 
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dresses.    Ill,  Navigation,  p.  456,  London,  1891. 
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London,  1902. 
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48  WATER  WAVES 

36.  Lyman,  C.  S.     A  New  Form  of  Wave  Apparatus.     Jour,  of  the  Frank- 

lin Institute.     LXXXVI,  187-194,  1868. 

37.  Stokes,    George    G.\briel.     On    the   Theory   of   Oscillatory   Waves. 

Mathematical  and  Physical  Papers.     I,  198,  208,  1880. 

38.  CiALDi,  Alessandro.     Sul  Moto  Ondoso  del  Mare  e  su  le  Correnti  di 

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dynamics.    Mathematical  and  Physical  Papers.     I,  164-165,  1880. 

41.  Emy,  a.  R.     Du  Mouvement  des  Ondes  et  des  Travaux  Hydrauliques 

Maritimes,  p.  49,  Paris,  1831. 

42.  Cialdi,  Alessandro.     Sul  Moto  Ondoso  del  Mare  e  su  le  Correnti  di 

esso.     695  pp.,  Rome,  1866. 

43.  Cornaglia,  p.     Sul  Regime  delle  Spiagge  e  sulla  Regolazione  dei  Porti. 

569  pp.,  Turin,  1891. 

Review,  Nature,  XLV,  362,  1892. 

44.  Thoulet,  J.     Oceanographie  Dynamique,  p.  54,  Paris,  1896. 

45.  Caligny,  a.  de.     Oscillations  de  I'Eau,  pp.  195-197,  Paris,  1883. 

46.  Bremontier,  N.  T.     Recherches  sur  le  Mouvement  des  Ondes.     122 

pp.,  Paris,  1809. 

47.  Emy,  A.  R.     Du  Mouvement  des  Ondes  et  des  Travaux  Hydrauhques 

Maritimes,  p.  17,  Paris.  1831. 

48.  Airy,   G.   B.     On  Tides  and  Waves.      Encyclopaedia   Metropolitana. 

V,  294,  1848. 
Fleming,  J.  A.     Waves  and  Ripples  in  Water,  Air,  and  ^ther,  p.  11, 
London,  1902. 
49     White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  199, 
London,  1900. 

50.  Cornish,   Vaughan.     Waves   of  the  Sea  and   Other  Water  Waves, 

p.  142,  Chicago,  1911. 

51.  Rankin,  W.  J.  M.     On  the  Exact  Form  of  Waves  near  the  Surface  of 

Deep  Water.     Philosophical  TraiLsactions  of  the  Royal  Society  of 
London.     CLIII,  Pt.  I,  127,  1863. 

52.  Fenneman,  N.  M.     Development  of  the  Profile  of  Equilibrium  of  the 

Subaqueous  Shore  Terrace.     Jour,  of  Geol.    X,  4,  1902. 

53.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  pp.  55, 
123,  Washington,  1904. 

54.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  213, 

London,  1900. 

55.  Gaillard,   D.   D.     Wave   Action  in  Relation  to  Engineering  Struc- 

tures.    Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31, 
pp.  36,  55,  Washington,  1904. 

56.  Stokes,    George    Gabriel.     On   the   Theory   of   Oscillatory   Waves. 

Mathematical  and  Physical  Papers.     I,  197-229,  1880. 

57.  Ste\t:nson,    Thomas.     The    Design   and    Construction   of   Harbours. 

3rd  edition,  pp.  78,  79,  Edinburgh,  1886. 


REFERENCES  49 

58.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,    Professional   Paper   No.  31,    pp. 
110-114,  Washington,  1904. 

59.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  p.  135, 

Chicago,  1911. 

60.  Krummel,  Otto.     Handbuch  der  Ozeanographie.     II,  Die  Bewegungs- 

formen  des  Meeres,  p.  112,  Stuttgart,  1911. 

61.  Hagen,  G.     Handbuch  der  Wasserbaukunst.     3.  Teil.  Das  Meer.  I, 

pp.  19,  86,  Berlin,  1863. 

62.  Russell,  J.  Scott.       Report  on  Waves,  made  to  the  Meetings  in  1842 

and  1843. 

Report  of  the  British  Association.     XIV,  371,  1844  (1845). 

63.  Bazin,  Henri.   Recherches  Experimentales  sur  la  Propagation' des  Ondes. 

Mem.  de  I'Acad.  des  Sciences  de  I'Inst.  de  France.    XIX,  518,  1865. 

64.  Russell,  J.  Scott.     Report  of  the  Committee  on  Waves,  appointed 

by  the  British  Association  at  Bristol  in  1836,  etc. 

Report  of  the  British  Association.     VII,  451,  1837  (1839). 
Russell,  J.  Scott.     Report  on  Waves,  made  to  the  Meetings  in  1842 
and  1843. 

Report  of  the  British  Association.     XIV,  371,  1844  (1845). 

65.  Cornish,  Vaughan.      Waves  of  the  Sea  and  Other  Water  Waves,  p. 

170,  Chicago,  1911. 

66.  Stevenson,    Thomas.     The   Design   and    Construction   of   Harbours. 

3rd  Edition,  pp.  77-78,  Edinburgh,  1886. 

67.  CiALDi,  Alessandro.     Sul  Moto  Ondoso  del  Mare  e  su  le  Correnti  di 

esso,  pp.  145-157,  Rome,  1866. 

68.  Thoulet,  J.     Oceanographie  Dynamique,  p.  51,  Paris,  1896. 

69.  KRtJMMEL,  Otto.     Handbuch  der  Ozeanographie.     II.    Die  Bewegimgs- 

forem  des  Meeres,  p.  Ill,  Stuttgart,  1911. 

70.  Ibid.,  p.  112. 

71.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

*  Corps  of  Engineers  U.   S.  Army,  Professional  Paper  No.   31,   pp. 
114-123,  Washington,  1904. 

72.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  pp. 

132,  133,  Chicago,  1911. 

73.  Ibid.,  pp.  Ill,  133. 

Cornish,  Vaughan.  On  the  Dimensions  of  Deep  Sea  Waves,  and 
their  Relations  to  Meteorological  and  Geographical  Conditions. 
Geographical  Jour.     XXIII,  643,  London,  1904. 

74.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.  67, 
Washington,  1904. 

75.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  pp. 

128,  129,  Chicago,  1911. 

76.  Weber,   Ernst   Heinrich   and   Wilhelm.     Wellenlehre   auf   Experi- 

mente  Gegriindet,  p.  25,  Leipzig,  1825. 

77.  Cornish,   Vaughan.     Waves   of   the  Sea  and  Other  Water  Waves, 

p.  106,  Chicago,  1911. 


50  WATER  WA\^S 

78.  Ste-\'t:nson,    Thomas.     The    Design   and   Construction   of   Harbours. 

3rd  Edition,  p.  29,  Edinburgh,  1886. 

79.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.  69, 
Washington,  1904. 

80.  Bois,  CoupvENT  DES.     Memoire  sur  la  Hauteur  des  Vagues  a  la  Sur- 

face des  Oceans.     Comptes  Rendus  de  I'Acad.  des  Sciences.    LXH, 
pp.  86-87,  1866. 

81.  Cornish,   Vaughan.     On  the   Dimensions  of  Deep  Sea  Waves,   and 

their    Relations    to    Meteorological    and    Geographical    Conditions. 
Geographical  Jour.     XXIII,  636,  London,  1904. 

82.  Stevenson,    Thomas.     The    Design    and    Construction   of    Harbours. 

3rd  Edition,  pp.  34,  35,  Edinburgh,  1886. 

83.  Cornish,  Vaughan.      Waves  of  the  Sea  and  Other  Water  Waves,  p. 

67,  Chicago,  1911. 

84.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.  82, 
Washington,  1904. 

85.  CoRNTiSH,    Vaughan.     Waves   of   the   Sea   and   Other   Water   Waves, 

pp.  33,  40,  Chicago,  1911. 

86.  ScoRFSBY,  William.     On  Atlantic  Waves,  their  Magnitude,  Velocity, 

and  Phenomena. 

Report  of  British  Association  for  1850,  Pt.  II,  p.  28,  1851. 

87.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  pp. 

53,  60,  Chicago,  1911. 

88.  ScoRESBY,    William.     On  Atlantic  Waves,  their  Magnitude,  Velocity, 

and   Phenomena.       Report  of  British  Association  for  1850,  Pt.  II, 

p.  28,  1851. 
Cornish,   Vaugh.an.     Waves  of  the  Sea  and  Other  Water  Waves, 

p.  60,  Chicago,  1911. 
Cornish,   Vaughan.     On  the   Dimensions  of  Deep  Sea  Waves,   and 

their   Relations    to    Meteorological    and    Geographical    Conditions. 

Geographical  Jour.     XXIII,  627,  London,^904., 

89.  CoR^^SH,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  p. 

62,  Chicago,  1911. 

90.  Ibid.,  pp.  74-77. 

91.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  212, 

London,  1900. 

92.  Gaillard,    D.   D.     Wave  Action  in  Relation   to   Engineering  Struc- 

tures.    Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31, 
pp.  76-79,  Washington,  1904. 

93.  Abercromby,  Ralph.     Observations  on  the  Height,  Length,  and  Ve- 

locity of  Ocean  Waves.     Philosophical  Magazine,  XXV,  269,  1888. 

94.  Airy,   G.   B.     On  Tides  and  Waves.      Encyclopaedia  Metropohtana, 

V,  351,  1848. 

95.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.  85, 
Washington,  1904. 


REFERENCES  51 

96.  Cornish,  Vaughan.     On  the  Dimensions  of  Deep  Sea  Waves,   and 

their  Relations  to  Meteorological  and  Geographical  Conditions.    Geo- 
graphical Jour.     XXIII,  626,  London,  1904. 

97.  Cornish  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  pp. 

96-101,  Chicago,  1911. 
Cornish,  Vaughan.     On  the  Dimensions  of  Deep  Sea  Waves,  and  their 
Relations  to  Meteorological  and  Geographical  Conditions.     Geographi- 
cal Jour.     XXIII,  627-633,  London,  1904. 

98.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Armj^,   Professional  Paper  No.  31,  p.  57, 
Washington,  1904. 

99.  Ibid.,  p.  70. 

100.  Paris,  A.     Observations  sur  I'Etat  de  la  Mer  RecueilHes  a  bord  du 

Dupleix  et  de  la  Minerve  (1867-70).     Revue  Maritime  et  Coloniale. 
XXXI,  121,  1871. 

101.  Schott,  Gerhard.     Uber.  die   Dimensionen  der  Meereswellen.     Fest- 

schrift Ferdinand  Freiherrn  von  Richthofen  zum  Sechzigsten  Geburts- 
tag,  p.  250,  Berlin,  1893. 

102.  White,    W.    H.     Manual    of    Naval   Architecture.     5th    Edition,    pp. 

213-214,  London,  1900. 

103.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  pp. 

30,  34,  Chicago,  1911. 

104.  Ibid.,  p.  92. 

105.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  211, 

London,  1900. 

106.  Bois,  CouPVENT  DES.     Memoirc  sur  la  Hauteur  des  Vagues  a  la  Surface 

des  Oceans.     Comptes  Rendus  de  I'Acad.  des  Sciences.    LXII,  p.  83, 
1866. 

107.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  p. 

73,  Chicago,  1911. 

108.  ScoRESBY,  William.     On  Atlantic  Waves,  their  Magnitude,  Velocity, 

and  Phenomena.      Report  of  British  Association  for  1850.     Pt.  II, 
29,  1851. 

109.  Cornish,    Vaughan.     Waves   of   the   Sea   and   Other   Water   Waves, 

pp.  70,  82,  Chicago,  1911. 

110.  Ibid.,  pp.  88-94. 

Cornish,  Vaughan.  On  the  Dimensions  of  Deep  Sea  Waves,  and  their 
Relations  to  Meteorological  and  Geographical  Conditions.  Geo- 
graphical Jour.     XXIII,  627,  London,  1904. 

111.  Antoine,  Ch.     Des  Lames  de  Haute  Mer,  p.  3,  Paris,  1879. 

112.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  j).  87, 

Chicago,  1911. 

113.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  205, 

London,  1900. 

114.  Airy,   G.   B.      On  Tides  and  Waves.      Encyclopedia   Metropolitana. 

V,  292,  1848. 

115.  Fleming,  J.  A.     Waves  and  Ripples  in  Water,  Air,  and  ^Ether,  p.  10, 

London,  1902, 


52  WATER  WAVES 

116.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  204, 

London,  1900. 

117.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.  38, 
Washington,  1904. 
lis.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  204, 
London,  1900. 

119.  FLE\nNG,  J.  A.     Waves  and  Ripples  in  Water,  Air,  and  ^Ether,  p.  11, 

London,  1902. 

120.  White,  W.  H.     Manual  of  Naval  Architecture.     5th  Edition,  p.  205, 

London,  1900. 

121.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.  44, 
Washington,  1904. 

122.  IHd.,  pp.  97-103. 

123.  Airy,  G.  B.     On  Tides  and  Waves.     Encyclopedia  Metropolitana.     V, 

292,  1848. 

124.  Russell,  J.  Scott.     Report  on  Waves  made  to  the  Meetings  in  1842 

and  1843. 

Report  of  the  British  Association.     XIV,  325,  1844  (1845). 

125.  Airy,  G.  B.     On  Tides  and  Waves.     Encyclopedia  MetropoUtana. 

V,  346,  1848. 

126.  Lagrange.     Mechanique  Analitique,  p.  491,  Paris,  1788. 

127.  Caligny,  a.  de.     Oscillations  de  I'Eau,  p.  199,  Paris,  1883. 

128.  Krummel,     Otto.     Handbuch    der    Ozeanographie.     II.   Die    Bewe- 

gungsformen  des  Meeres,  p.  29,  Stuttgart,  1911. 

129.  Ibid.,  p.  152. 

130.  Russell,  J.  Scott.     Report  of  the  Committee  on  Waves,  appointed 

by  the  British  Association  at  Bristol  in  1836,  etc. 

Report  of  the  British  Association.     VII,  417-496,  1837  (1838). 
Russell,  J.  Scott.     Report  on  Waves,  made  to  the  Meetings  in  1842 
and  1843. 

Report  of  the  British  Association.     XIV,  311-390,  1844  (1845). 

131.  Russell,  J.  Scott.     Report  on  Waves,  made  to  the  Meetings  in  1842 

and  1843. 

Report  of  the  British  Association.     XIV,  340-347,  1844  (1845). 

132.  Ibid.,  pp.  340,  354. 

133.  Ibid.,  pp.  325-328. 

134.  Krummel,  Otto.     Handbuch  der  Ozeanographie.     II.   Die  Bewegungs- 

formen  des  Meeres,  pp.  149-150,  Stuttgart,  1911. 
Wheeler,  W.  H.     A  Practical  Manual  of  Tides  and  Waves,  p.  -131, 
London,  1906. 
135    Wheeler,  W.  H.     A  Practical  Manual  of  Tides  and  Waves,  p.  57, 
London,  1906. 

136.  Caligny,  A.  de.     Oscillations  de  I'Eau,  pp.  191-211,  Paris,  1883. 

137.  Hunt,  A.  R.     On  the  Action  of  Waves  on  Sea-Beaches  and  Sea-Bot- 

toms.    Proc.  Roy.  Dubhn  Soc,  N.  S.     IV,  251-259,  1884. 


REFERENCES  53 

138.  Russell,  J.  Scott.     Report  on  Wave.s,  made  to  the  Meetings  in  1842 

and  1843. 

Report  of  the  British  Association.     XIV,  372-373,  1844  (1845). 

139.  HoBBS,  Wm.  H.     Origin  of  Ocean  Basins  in  the  Light  of  the  New  Seis- 

mology.    Bull.  Geol.  Soc.  Amer.     XVIII,  242,  1907. 

140.  Krummel,  Otto.     Handbuch  dcr  Ozeanographie.     II.    Die  Bewegungs- 

formen  des  Meeres,  p.  133,  Stuttgart,  1911. 

141.  Russell,  J.  Scott.     Report  on  Waves,  made  to  the  Meetings  in  1842 

and  1843. 

Report  of  the  British  Association.     XIV,  323,  1844  (1845). 

142.  Reid,   H.   F.     Earthquake  Sea  Waves.     Unpublished  paper  read  at 

Princeton  Meeting  of  Geological  Society  of  America.     December, 
1913. 

143.  Russell,  J.  Scott.     Report  on  Waves,  made  to  the  Meetings  in  1842 

and  1843. 

Report  of  the  Briti.sh  Association.     XIV,  323,  1844  (1845). 

144.  Weber,    Ernst   Heinrich   and   Wilhelm.     Wellenlehre   auf   Experi- 

mente  Gegriindet.     433  pp.,  Leipzig,  1825. 

145.  Krijmmel,  Otto.     Handbuch  der  Ozeanographie.     II.    Die  Bewegungs- 

formcn  des  Meeres.     766  pp.,  Stuttgart,  1911. 

146.  Thoulet,  J.     Oceanographie  Dynamique.     131  pp.,  Paris,  1896. 

147.  Krijmmel,     Otto.     Handbuch    der    Ozeanographie.     II.    Die     Bewe- 

gungsformen  des  Meeres,  pp.  136-137,  Stuttgart,  1911. 

148.  Ibid.,  p.  141. 

149.  Ibid.,  p.  148. 

150.  Ibid.,  p.  149. 

Wheeler,  W.  H.  A  Practical  Manual  of  Tides  and  Waves,  p.  131, 
London,  1906. 

151.  Krummel,  Otto.     Handbuch  der  Ozeanographie.     II.    Die  Bewegungs- 

foniien  des  Meeres,  p.  149,  Stuttgart,  1911. 

152.  Ibid.,  p.  139. 

153.  Ibid.,  p.  148. 

Gaillard,  D.  D.  Wave  Action  in  Relation  to  Engineering  Structures. 
Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.  91, 
Washington,  1904. 

154.  Russell,  J.  Scott.     [On  local  changes  of  tide  heights  and  on  the  char- 

acter of  the  tide-wave.]     Min.  Proc.  Inst.  Civ.  Eng.     VII,  364,  1848. 

155.  Wheeler,  W.  H.     The  Sea  Coast;    Destruction:    Littoral  Drift:    Pro- 

tection, p.  8,  London,  1902. 

156.  Haupt,  L.  M.     Discussion  on  the  Dynamic  Action  of  the  Ocean  in 

Building  Bars.     Proc.  Am.  Phil.  Soc.    XXVI,  pp.  147,  148,  155,  1889. 

157.  Marsh,  Geo.  P.      The  Earth  as  Modified  by  Human  Action,  p.  538, 

New  York,  1907. 

158.  Krijmmel,  Otto.     Handbuch  der  Ozeanographie.     II.    Die  Bewegungs- 

formen  des  Meeres,  p.  158,  Stuttgart,  1911. 

159.  Ibid.,  p.  166. 

160.  Harris,  R.  A.     Manual  of  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907.     Appendix  No.  6,  472-482,  1907. 


54  WATER  WAVES 

161.  Thoulet,  J. .  Oceanographie  Dynamique,  pp.  71-84,  Paris,  1896. 

162.  Dawson,    W.    Bell.     Illustrations    of    Remarkable    Secondary    Tidal 

Undulations  in  January,  1899,  as  Registered  on  Recording  Tide 
Gauges  in  the  Region  of  Nova  Scotia.  Trans.  Roy.  Soc.  Canada. 
2nd  Ser.,  V.  Sec.  Ill,  24,  1899. 

163.  Darwin,   G.   R.     The  Tides   and   Kindred  Phenomena  in  the  Solar 

System,  pp.  16-49,  London,  1898. 

164.  Sandstrom,  J.  W.     Dynamische  Versuche  mit  Meerwasser.     Annalen 

der  Hydrographie  und  Maritimen  Meteorologie.     XXXVI,  10,  1908. 

165.  Helland-Hansen,   Bjorn  and  Nansen,   Fridtjof.     The  Norwegian 

Sea,  p.  116,  Christiania,  1909. 

166.  Ekman,  V.  W.     On  Dead  Water.     The  Norwegian  North  Polar  Expedi- 

tion 1893-1896,  Scientific  Results.     V,  No.  15,  p.  42,  Christiania,  1906. 

167.  Helland-Hansen,  Bjorn  and  Nansen,  Fridtjof.    The  Norwegian 

Sea,  pp.  114-117,  Christiania,  1909. 

168.  Stokes,   George  Gabriel.      On   the  Theory  of  Oscillatory  Waves. 

Mathematical  and  Physical  Papers,  I,  212-219,  1880. 


CHAPTER  11 
THE  WORK   OF   WAVES 

Advance  Summary.  —  Water  waves,  whose  general  charac- 
teristics were  discussed  in  Chapter  I,  possess  energy  capable  of 
effecting  profound  changes  upon  the  margins  of  the  land  or 
upon  artificial  structures  with  which  they  may  come  into  con- 
tact. Geologist,  geographer,  and  engineer  must  each  concern 
himself  with  the  nature  and  magnitude  ,of  wave  energy,  and 
with  the  manner  in  which  waves  accomplish  their  work.  The 
layman  finds  the  destructive  energy  of  waves  a  source  of  inter- 
est and  wonder,  and  not  unnaturally  regards  the  meeting-place 
of  land  and  sea  as  one  of  the  most  fascinating  of  Nature's  lab- 
oratories. 

In  the  present  chapter  the  nature  of  wave  energy  is  first  dis- 
cussed, and  the  manner  of  wave  attack  upon  cliffs  and  sloping 
shores  is  briefly  treated.  It  is  then  shown  that  the  dynamic 
pressures  exerted  by  waves  may  be  measured  with  reasonable 
exactness,  and  calculated  and  measured  pressures  are  shown 
to  be  in  substantial  agreement.  Some  of  the  most  striking 
examples  of  damage  done  by  storm  waves  are  next  passed  in 
review,  in  order  that  the  reader  may  visualize  the  magnitude  of 
the  force  responsible  for  the  modification  of  shore  features  and 
the  manifold  methods  of  its  working.  In  order  to  determine 
which  parts  of  a  shore  or  what  artificial  structures  will  suffer 
most  from  wave  attack,  it  is  essential  to  know  precisely  what 
factors  control  wave  energy,  and  these  are  briefly  considered. 
A  process  of  "wave  refraction  "  is  shown  to  be  responsible  for 
the  concentration  of  wave  attack  upon  projecting  headlands 
and  for  the  comparative  immunity  of  shores  about  the  heads 
of  bays.  In  conclusion,  attention  is  directed  to  the  vitally  im- 
portant question  as  to  how  far  below  the  water  surface  wave 
action  may  be  appreciable. 

Wave  Energy.  —  It  can  readily  be  shown  that  a  wave  transmits 
energy  along  the  surface  of  a  water  body,  and  delivers  this  energy 
on  the  beach  or  against  some  artificial  obstacle.     When  a  ship  is 

55 


56  THE  WORK  OF  WAVES 

propelled  through  the  water,  a  wave  is  pushed  up  by  the  bow. 
It  took  a  certain  amount  of  energy  to  raise  this  mound  of  water, 
and  that  amount  was  taken  away  from  the  energy  of  the  moving 
vessel,  thereby  causing  the  vessel's  motion  to  be  retarded.  The 
wave  passes  over  the  surface  of  the  water;  and  if  it  finally  dashes 
upon  some  beach,  the  energy  there  expended  is  the  same  energy 
imparted  by  the  moving  boat,  less  a  small  amount  lost  through 
friction. 

The  mere  spreading  apart  of  the  water  by  a  vessel's  bow  does 
not  require  the  expenditure  of  energy.  If  it  were  not  for  other 
causes  of  resistance,  a  ship  once  started  through  the  water  would 
move  on  forever,  unimpeded  by  the  pushing  apart  of  the  water 
in  front.  The  common  idea  that  a  vessel's  bow  is  made  sharp 
so  that  it  may  cut  into  the  water  like  a  wedge  and  more  easily 
push  it  out  of  the  way,  is  erroneous.  No  part  of  the  resistance 
to  a  ship's  motion  arises  directly  from  the  pushing  of  water  to 
either  side  by  the  bow^  A  great  deal  of  resistance  does  arise, 
however,  from  the  fact  that  energy  is  used  up  in  making  waves, 
and  one  object  of  the  naval  architect  is  to  design  a  vessel  of  such 
form  that  it  will  produce  the  fewest  and  smallest  waves  possible. 

The  energy  of  a  wave  depends  upon  its  length  and  height, 
and  is  of  two  types :  the  kinetic  energy  due  to  the  orbital  move- 
ment of  the  water  particles;  and  the  potential  energy  due  to  the 
fact  that  the  center  of  gravity  of  the  mass  of  water  composing 
a  wave  is  raised  slightly  above  the  position  it  occupies  when  the 
water  is  at  rest.  It  can  be  shown  that  the  two  types  of  energy 
are  exactly  equal  in  amount;  in  other  words,  the  energy  of  a 
wave  is  half  kinetic  and  half  potential.  Since  we  know  that  a 
cubic  foot  of  sea  water  weighs  about  64  pounds,  it  is  easy  to 
calculate  the  total  energy  of  either  shallow-water  or  deep-water 
waves  in  foot-tons  per  linear  foot  of  wave  crest.  The  formulae 
employed  in  such  calculations  are  too  complex  for  discussion 
here,  but  may  be  found  in  Gaillard's  treatise^,  and  similar  works. 

During  the  advance  of  a  deep-water  oscillatory  wave  one-half 
of  the  total  wave  energy  is  transmitted  forward  with  the  wave 
form.  The  energy  of  shallow-water  oscillatory  waves  is  from  1 
per  cent  to  1 1  per  cent  less  than  the  energy  of  deep-water  waves 
of  equal  length  and  height,  but  just  as  in  the  case  of  deep-water 
waves  one-half  the  total  wave  energy  is  transmitted  onward. 
In  both  cases  it  is  the  potential  energy  which  is  thus  carried 


NATURE  OF  WAVE  ATTACK  57 

forward  with  the  wave^.  In  the  wave  of  translation  the  energy 
is  also  partly  kinetic  and  partly  potential ;  but  as  this  wave  leaves 
still  water  behind  it,  at  the  original  level,  the  entire  wave  energy 
must  pass  forward  with  the  wave  form.  We  have  already  seen 
that  when  oscillator}^  waves  pass  into  water  which  shoals  very 
gradually,  they  are  slowly  transformed  into  waves  of  translation, 
or  at  least  acquire  some  of  the  characteristics  of  such  waves. 
From  this  it  follows  that  an  oscillatory  wave  ma,y,  by  changing 
into  a  wave  of  translation,  deliver  at  the  shore  all,  or  nearly  all, 
its  energy*.  This  may  help  to  explain  the  fact  that  the  blows 
of  storm  waves  against  a  cliff  or  sea  wall  often  exceed  in  vio- 
lence the  available  energy  calculated  for  the  waves  on  the  as- 
sumption that  they  are  waves  of  oscillation. 

Nature  of  Wave  Attack.  —  The  nature  of  the  force  exerted  by 
a  wave  upon  any  obstacle,  such  as  a  cliff  or  beach,  depends  in 
part  upon  the  type  of  wave  and  its  condition  atJh&jUQment  oi 
collision  with  the  obstacle!  If  an  unbroken  oscillatory  wave 
stri^es^aTvertical  wall  or  cliff  the  base  of  which  reaches  down  to 
deep  water,  the  wave  is  reflected  back.  At  the  instant  of  con- 
tact the  crest  of  the  wave  rises  to  twice  its  normal  height  and 
the  cliff  is  subjected  to  the  hydrostatic  pressure  of  this  unusually 
high  water  column.  The  absence  of  any  forward  thrust  of  the 
water  mass  under  these  conditions  is  shown  by  the  behavior  of 
boats  which  have  been  observed  to  rise  and  fall  with  successive 
waves  without  touching  the  vertical  wall  only  a  few  feet  distant. 
Hagen*  concludes  that  under  such  circumstances  debris  must 
accumulate  at  the  base  of  the  wall  and  that  therefore  the  preju- 
dice against  vertical  sea  walls  and  harbor  walls,  based  on  the  fear 
of  undermining  by  wave  action,  is  ill-founded. 

A  wave  of  translation  striking  a  vertical  wall  or  cliff  under  the 
same  circumstances  is  also  reflected;  but  it  delivers  against  the 
cliff  a  vigorous  push  due  to  the  forward  thrust  of  the  whole  mass 
of  the  wave,  in  addition  to  subjecting  the  obstruction  to  hydro- 
static pressure.  Stevenson"  found  that  "  oscillatory  waves  become 
waves  of  translation  when  they  reach  the  unfinished  part  of  a  ver- 
tical sea  wall,  and  that  they  then  exert  a  force  nearly  6  times 
greater  than  if  they  had  remained  waves  of  oscillation."  If  either 
type  of  wave  breaks  just  before  reaching  the  cliff,  in  such  manner 
that  the  forward  falling  crest  of  the  wave  strikes  the  cliff  face,  the 
only  force  exerted  is  that  due  to  the  forward  motion  of  the  water 


58  THE  WORK  OF  WAVES 

particles.  This  motion  may  exceed  the  velocity  of  the  wave 
itself  at  the  time  of  breaking,  the  crest  shooting  forward  beyond 
the  main  body  of  the  wave  as  it  falls.  When  an  oscillatory 
wave  breaks  a  short  distance  out  in  front  of  the  cliff,  so  that 
the  forward  pitching  crest  does  not  strike  the  cliff,  but  plunges 
into  the  water  at  its  base,  the  regular  orbital  motion  is  destroyed 
and  a  "  whirlpool  turbulence  "  is  produced,  the  forces  of  which 
are  not  easily  analyzed.  In  a  similar  manner,  if  a  wave  of 
translation  breaks  just  before  reaching  a  cliff,  it  "  becomes  a 
surge  or  broken  foam,  a  disintegrated  heap  of  water  particles, 
having  lost  all  continuity."  The  moving  waters  of  the  surge  or 
whirlpool  turbulence  may  exert  considerable  dynamic  pressure 
on  the  base  of  the  cliff,  and  some  hydrostatic  pressure,  depend- 
ing on  the  height  to  which  the  water  rises.  When  either  oscil- 
latory waves  or  waves  of  translation  break  far  out  from  the  base 
of  the  cliff,  smaller  waves  of  translation  may  traverse  the  inter- 
vening water  and  operate  upon  the  cliff  in  the  manner  already 
described. 

On  a  sloping  shore  of  fairly  steep  inclination,  oscillatory  waves 
may  arrive  almost  at  the  beach  before  losing  their  essential 
characters.  When  such  a  wave  breaks  the  falling  crest  dashes 
down  upon  the  water  which  is  returning  seaward  from  the  swash 
of  the  preceding  wave.  The  falhng  wave  crest  thus  strikes  a 
cushion  of  moving  water  which  may  be  of  considerable  thickness. 
A  zone  of  great  confusion  is  thus  produced,  the  force  of  the  wave 
is  largely  dissipated,  and  part  of  its  volume  augments  the  sheet 
of  water  mo\:Jng  seaward,  while  a  larger  part  starts  up  the  beach. 
Almost  instantly  the  remainder  of  the  breaking  wave  over- 
takes the  zone  of  disturbance,  the  forward  oscillation  under  the 
crest  checking  and  possibly  reversing  the  seaward  motion  of 
the  bottom  water,  while  the  landward  moving  water  is  enor- 
mously augmented  in  volume.  At  the  same  time  the  orbital 
motion  of  the  water  is  largely  destroyed,  and  in  the  form  of  a 
confused  mass  it  rushes  up  the  beach  until  stopped  by  gravity 
and  friction,  when  it  flows  back  with  gradually  increasing  velocity 
to  meet  the  next  oncoming  wave.  Under  these  conditions  much 
of  the  energy  of  the  wave  is  consumed  by  friction  in  the  turbulent 
waters,  while  another  part  is  expended  in  the  impact  of  the 
falhng  crest  upon  the  bottom  wherever  the  sheet  of  seaward 
moving  water  is  effectively  pierced.     The  beach  itself  is  affected 


NATURE  OF  WAVE  ATTACK 


59 


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60  THE  WORK  OF  WAVES 

mainly  by  the  sheet  of  water  which  is  propelled  up  the  slope 
by  the  remnant  of  the  wave's  energy  of  motion,  and  which  re- 
turns under  the  action  of  gravity. 

It  should  be  noted  that  after  the  oscillatory  wave  breaks,  the 
confused  mass  of  water  propelled  up  the  slope  of  the  beach  may 
be  regarded  as  an  irregular  type  of  wave  of  translation.  When 
a  typical  wave  of  translation  breaks  immediately  at  the  foot  of 
the  beach,  its  falhng  crest  must  also  meet  the  backward  flowing 
water  cast  up  by  the  preceding  wave,  and  give  rise  to  much  the 
same  phenomena  as  the  breaking  oscillatory  wave. 

On  a  coast  bordered  by  water  so  shallow  that  large  oscillatory 
waves  are  broken  some  distance  out  from  the  shoreline,  waves 
of  translation  and  small  oscillatory  waves  alone  may  reach  the 
beach.  If  the  beach  slopes  very  gradually  under  water,  there 
may  be  a  secondary  line  of  surf  a  short  distance  out  where  these 
waves  break,  and  the  amount  of  wave  energy  which  finally 
reaches  the  beach  itself  may  be  quite  insignificant.  On  the 
other  hand,  if  the  water  between  the  shoreline  and  the  zone 
where  the  great  oscillatory  waves  break  is  of  fairly  constant 
depth,  and  the  shore  rises  fairly  abruptly  at  the  inner  margin 
of  the  shallow,  waves  of  translation  of  considerable  size  may 
deliver  their  whole  energy  upon  the  beach.  The  latter  is  then 
subjected  to  the  static  pressure  due  to  the  wave  height,  and  the 
dynamic  force  of  the  rapidly  moving  water  particles. 

When  a  wave  comes  in  contact  with  a  vertical  or  very  steep  wall 
or  cliff,  a  relatively  small  portion  of  the  wave  mass  may  be  shot 
upward  (Plates  IV  and  Y).  It  appears  that  under  these  circum- 
stances the  energy  of  a  large  portion  of  the  wave  is  suddenly 
communicated  to  the  smaller  water  mass.  The  result  is  that  the 
velocity  of  this  mass  may  be  very  great,  and  it  ma}^  dehver  a  blow 
of  terrific  force  upon  a  small  area.  Overhanging  chffs  or  projec- 
tions from  cliff  faces,  the  roofs  of  sea  caves,  and  other  masses 
of  rocks  favorably  situated  may  be  subjected  to  blows  from 
below  which  have  the  strength  of  a  battering  ram.  The  energy 
expended  is  the  kinetic  energy  due  to  the  swift  motion  of  the 
water  masses. 

Masses  of  water  shot  into  the  air  in  the  manner  just  described 
may  encounter  no  obstacle  in  their  upward  flight,  but  may 
descend  upon  the  'evel  summit  or  sloping  face  of  a  cliff,  the  sur- 
face of  a  beach,  or  some  artificial  structure.     Such  falhng  masses 


NATURE  OF  WAVE  ATTACK 


61 


Plate  V 


Photo  uy  A.  M.  Croinack, 

Water  forced  vertically  upward  by  wave  breaking  against  sea  tvall  at 
Scarborough,  England. 


62 


THE  WORK  OF  WAVES 


of  water  are  capable  of  executing  considerable  damage  because 
of  the  great  energy  they  acquire  by  descending  with  the  ever- 
increasing  velocity  due  to  gravitation. 

Wave  Dynamometer.  —  Stevenson  has  shown  that  the  action 
of  a  wave  is  not  at  all  like  the  sudden  impact  of  a  hard  body,  but 
is  analogous  to  the  steady  pressure  of  a  current,  because  the  wave 
acts  with  a  continuous  pressure  for  an  appreciable  length  of 
time'^.  It  follows  from  this  that  if  waves  are  allowed  to  come 
against  a  vertical  plate  which  has  a  spring  back  of  it,  and  if  the 


Fig.  11.  —  Stevenson's  Wave  Dynamometer. 
DEED  is  a  cast-iron  cylinder,  bolted  to  the  rock  by  the  flanges  at  G.  AA  is 
an  iron  dislv  against  which  the  waves  impinge,  fastened  to  guide  rods  BB, 
which  pass  through  holes  in  the  plate  CC.  When  waves  strike  the  disk 
AA,  rings  of  leather  TT  are  moved  along  the  guide  rods,  registering  the 
extent  to  which  the  spring  is  lengthened.  LL  is  a  door  opened  for  the 
purpose  of  reading  the  instrument. 

change  in  length  of  the  spring  due  to  the  pressure  against  the  plate 
is  determined,  we  shall  have  a  proper  measure  of  the  dynamic 
pressure  exerted  by  the  wave.  Stevenson^  devised  such  an  instru- 
ment, called  a  dynamometer,  and  was  the  first  man  to  measure 
the  force  of  waves.  Gaillard  confirmed  Stevenson's  results,  hut 
pointed  out  that  the  spring  dynamometer  measures  only  the  dy- 
namic pressure  of  the  moving  water  in  the  wave,  and  gives  no 
information  as  to  the  static  pressure  resulting  from  the  weight 
of  the  water  mass.  This  is  due  to  the  fact  that  static  pressure  is 
just  as  great  on  the  back  of  the  plate  as  on  the  front,  and  therefore 
produces  no  effect  on  the  spring.     He  therefore  designed  a  dia- 


MEASUREMENTS  OF   WAVE  ENERGY  63 

phragm  dynamometer  having  a  sheet  of  rubber  stretched  over 
one  end  of  a  short  iron  cyUnder,  the  other  end  being  closed  by 
an  iron  plate.  Pressures  due  to  the  waves  thus  affect  but  one 
side  of  the  instrument  by  pushing  in  the  rubber  diaphragm,  and 
the  magnitudes  of  the  pressures  are  determined  by  means  of 
a  gauge  attached  to  the  cylinder.  To  measure  the  static  pres- 
sure due  to  the  column  of  water  in  the  wave  the  instrument  is 
placed  with  its  face  horizontal  and  upward  at  the  desired  depth 
in  the  water.  When  placed  with  its  face  vertical  so  as  to  receive 
the  full  impact  of  the  advancing  wave  the  instrument  records 
both  dynamic  and  static  pressures^. 

Measurements  of  wave  force  with  dynamometers  indicate  that 
the  static  pressures  exerted  by  waves  are  considerably  less  than 
their  dynamic  pressures.  On  Lake  Superior,  Gaillard  found  the 
static  pressure  of  a  wave  10.5  feet  high  and  150  feet  long,  to  be 
3.23  lbs.  per  square  inch,  or  about  450  lbs.  per  square  foot,  the 
dynamometer  being  9  feet  below  the  wave  crest.  The  dy- 
namic pressures  of  waves  10  feet  high  and  150  feet  long  varied 
from  460  to  965  lbs.  per  square  foot  on  a  dynamometer  placed 
about  a  foot  higher  than  that  for  the  measurement  of  static 
pressures^*^.  Adequate  observations  of  wave  pressures  by  means 
of  suitable  dynamometers  have  not  yet  been  made,  those  for 
static  pressures  being  especially  insufficient  in  number. 

Measurements  of  Wave  Energy.  —  In  order  to  gain  some  con- 
ception of  the  enormous  power  of  waves  we  have  only  to  consider 
the  theoretical  pressures  calculated  for  waves  of  different  size,  the 
actual  pressures  recorded  by  dynamometers  on  exposed  coasts,  or 
the  damage  to  harbor  works  done  by  storm  waves.  Gaillard  has 
calculated  that  a  wave  10  feet  high  and  100  feet  in  length  may 
strike  an  obstruction  with  a  pressure  of  1675  lbs.  per  square  foot, 
while  a  wave  12  feet  high  and  200  feet  long  should  exert  a  maximum 
dynamometer  pressure  of  2436  lbs.  per  square  foot.  The  total 
theoretical  energy  of  such  a  wave  is  109  foot-tons  for  every 
linear  foot  of  wave  crest.  Great  ocean  waves  such  as  those 
which  destroyed  part  of  the  breakwater  at  Wick,  Scotland,  in 
1872,  if  we  assume  a  height  of  42  feet  and  a  length  of  500  feet, 
should  produce  a  pressure  of  6340  pounds  per  square  foot". 

Dynamometer  readings  show  that  during  storms  on  Lake 
Superior  the  waves  develop  a  force  of  from  1600  to  2500  lbs. 
per  square  foot^^     Stevenson  found  that  the  Atlantic  Ocean 


64 


THE  WORK  OF  WAVES 


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DAMAGE   BY   STORM   WAVES  65 

waves  near  the  island  of  Tyree  on  the  Scottish  coast  had  an 
average  force  of  611  lbs.  per  square  foot  during  the  summer 
months,  whereas  the  average  for  the  winter  months  was  2086 
lbs.,  or  more  than  three  times  as  great.  The  greatest  force 
recorded  at  this  point  was  6083  lbs.  or  practically  that  calculated 
theoretically  for  a  very  large  ocean  wave.  Another  reading  of 
5323  lbs.  was  secured.  On  the  east  coast  of  Scotland  pressures 
of  more  than  6000  lbs.  per  square  foot  were  recorded^^ 

Damage  by  Storm  Waves.  —  Such  enormous  pressures  are  cap- 
able of  producing  remarkable  results.  Stevenson  describes  an  in- 
stance in  which  a  block  of  stone  weighing  7|  tons  and  situated  20 
feet  above  sealevel  was  driven  before  the  waves  for  a  distance  of 
73  feet  over  rugged  ledges^^.  At  North  Beach,  Florida,  a  solid  block 
of  concrete  weighing  4500  lbs.  was  moved  12  feet  horizontally  and 
turned  over  on  its  side,  while  a  second  block  weighing  3600  lbs.  and 
having  its  center  at  high-water  level  was  shifted  several  inches 
by  waves  which  were  not  over  4  feet  high.  During  a  severe 
storm  on  December  25,  1836,  stones  forming  part  of  the  break- 
waterjat  Cherbourg  and  weighing  nearly  7000  lbs.  were  thrown 
over  a  wall  20  feet  high  which  surmounts  the  stone  embank- 
ment. In  the  harbor  of  Cette  a  block  of  concrete  2500  cubic 
feet  in  volume  and  weighing  about  125  tons  was  shifted  more 
than  3  feet  from  its  original  position.  Perhaps  the  most  won- 
derful example  of  wave  work  is  that  accomplished  by  ocean 
storm  waves  upon  the  breakwater  at  Wick  in  December,  1872, 
and'  described  in  Stevenson's  treatise  on  "Harbors."  The 
seaward  end  of  this  breakwater  was  protected  by  a  monoUthic 
block  of  cement  rubble  45  feet  long,  26  feet  wide  and  11  feet  thick, 
and  weighing  more  than  800  tons,  resting  on  great  blocks  of 
stone  which  were  bound  solidly  to  the  monolith  by  iron  rods  3| 
inches  in  diameter  running  through  holes  in  the  stones  and 
embedded  in  the  cement  rubble.  The  entire  mass,  weighing 
1350  tons,  was  torn  from  its  place  by  the  waves  and  dropped 
inside  the  pier,  where  it  was  found  unbroken  after  the  storm 
subsided.  A  much  larger  mass  of  concrete  was  substituted  for 
the  one  removed,  the  new  block  having  a  volume  of  1500  cubic 
yards,  and  weighing  2600  tons.  In  1877  this  enormous  mass  was 
similarly  carried  away  by  the  waves^^. 

The  terrific  impact  which  a  wave  may  deliver  against  the 
face  of  a  vertical  wall  may  be  appreciated  from  the  fact  that  the 


66  THE  WORK  OF  WAVES 

facing  stones  of  the  Wick  breakwater,  having  the  same  density 
as  granite,  were  shattered  by  the  sea  in  February,  1872.  At 
Dunkirk  waves  from  the  narrow  southern  arm  of  the  North  Sea 
strike  the  coast  with  an  impact  which  causes  a  trembhng  of  the 
ground  more  than  a  mile  inland^^.  That  waves  have  the  power 
to  wrench  from  place  objects  situated  well  above  the  main  body 
of  the  wave  is  shown  by  the  effects  of  a  storm  upon  Dhuheartach 
lighthouse  on  the  west  coast  of  Scotland,  during  which  fourteen 
stones  weighing  2  tons  each  were  torn  from  their  positions  37  feet 
above  high  tide,  and  dropped  into  deep  water.  Cast-iron  lamp 
posts  on  the  pier  heads  at  Duluth,  located  19  feet  above  lake 
level,  have  repeatedly  been  broken  off  by  wave  action. 

The  lifting  power  of  waves  is  often  illustrated  by  damage  to  har- 
bor works.  At  North  Beach,  Florida,  a  block  of  concrete  weighing 
10|  tons  was  lifted  vertically  upward  three  inches  by  the  wave 
pressure  transmitted  through  crevices  below  the  mass.  During 
a  storm  on  Lake  Superior  a  mass  of  trap  rock  2  cubic  yards  in 
volume  and  weighing  about  4|  tons  was  raised  by  a  wave  from 
its  place  alongside  an  old  breakwater  at  Duluth  and  deposited 
on  the  surface  of  the  breakwater  some  5  or  6  feet  above  its  original 
position.  A  more  striking  example  occurred  at  Ymuiden  on  the 
coast  of  Holland,  when  a  concrete  block  weighing  20  tons  was 
lifted  12  feet  vertically  by  a  wave  and  deposited  on  a  pier  above 
high-water  level. 

Waves  deflected  upward  by  a  sloping  surface  may  accom- 
plish work  at  high  levels.  The  keeper  of  Trinidad  Head  light 
station,  on  the  Pacific  Coast,  reports  that  during  the  storm 
of  December  28,  1913,  the  waves  repeatedly  washed  over  Pilot 
Rock,  103  feet  high.  One  unusually  large  wave  struck  the 
cliffs  below  the  light  and  rose  as  a  solid  sea  apparently  to  the 
same  level  at  which  he  was  standing  in  the  lantern,  196  feet 
above  mean  high  water,  the  spray  rising  25  feet  or  more  higher. 
The  shock  of  the  impact  against  the  cliffs  and  tower  was  terrific, 
and  stopped  the  revolving  of  the  light.  Lake  Superior  waves 
reached  the  door  of  a  light-keeper's  dwelling  situated  140  feet 
back  from  the  water  and  60|  feet  above  it,  carrying  away  a 
board  walk  and  doing  other  minor  damage.  On  the  Bound 
Skerry  in  the  Shetland  Islands  blocks  of  stone  from  6  to  13  tons 
in  weight  have  been  forced  from  their  places  at  a  level  which  is 
70  to  75  feet  above  the  sea. 


DAMAGE  BY  STORM   WAVES  67 

The  destructive  power  of  the  masses  of  water  hurled  to  re- 
markable heights  by  breaking  waves  is  greater  than  one  might 
suppose.  At  the  Bell  Rock  Ughthouse  in  the  North  Sea  a  ground- 
swell,  without  the  aid  of  wind,  drove  water  to  the  summit  of  the 
tower  106  feet  above  high  tide,  and  l^roke  off  a  ladder  at  an 
elevation  of  80  feet.  A  bell  weighing  3  cwt.  was  broken  from 
its  place  in  the  Bishop  Rock  hghthouse,  100  feet  above  high 
water  mark,  during  a  gale  in  1860;  and  at  Unst,  in  the  Shetland 
Islands,  a  door  was  broken  open  at  a  height  of  195  feet  above 
the  sea.  The  keeper  of  Tillamook  Rock  lighthouse,  on  the  coast 
of  Oregon,  reports  that  in  the  winter  of  1902  the  water  of  waves 
was  thrown  more  than  200  feet  above  the  level  of  the  sea,  de- 
scending upon  the  roof  of  his  house  in  apparently  solid 
masses.  In  October  1912,  and  again  in  November  1913,  the 
panes  of  plate  glass  in  the  lantern  of  this  same  light,  132  feet 
above  mean  high  water,  were  broken  in  by  storm  waves. 

Great  damage  may  be  accomplished  by  the  falling  water.  Ac- 
cording to  Shield^^  "it  is  no  uncommon  occurrence  for  storm  waves, 
striking  a  vertical  breakwater  face,  to  throw  heavy  masses  of 
water  to  a  height  of  at  least  100  feet,  often  very  much  higher. 
Such  water  in  its  descent  on  reaching  the  roadway  of  the  break- 
water upon  which  it  falls,  will  have  attained  a  velocity  of  about 
80  feet  per  second,  or  nearly  double  the  velocity  and  four  times 
the  force  of  the  water  striking  the  face  of  the  breakwater."  Dur- 
ing a  severe  gale  at  Buffalo  in  December,  1899,  seventy  big  tim- 
bers, 12  X  12  inches  in  thickness,  12  feet  long,  and  10  feet  between 
supports,  were  broken  in  two  in  the  middle  by  the  impact  of  the 
falhng  water.  This  same  breakwater  was  further  damaged  a 
year  later  when  waves  breaking  against  it  were  hurled  from  75 
to  125  feet  into  the  air,  the  falling  water  crushing  the  big  timbers 
on  which  it  fell  as  though  they  had  been  pipestems. 

A  part  of  the  geological  work  accomplished  by  waves  is  due 
to  the  direct  pressure  exerted  upon  air  and  water  imprisoned  in 
crevices,  and  another  part  to  the  sudden  expansion  of  air  in 
crevices  and  pore  spaces  when  the  rapid  retreat  of  a  wave  creates 
a  partial  vacuum  outside.  The  effect  of  compressed  air  may  be 
inferred  from  the  fact  that  waves  coming  against  a  breakwater  in 
Buffalo  harbor  produced  such  high  pressure  upon  the  air  under  the 
concrete  shell  that  four  circular  plates  of  concrete,  3  feet  in  diam- 
eter, 6  inches  thick  and  weighing  530  lbs.  each,  serving  as  covers  to 


68  THE  WORK  OF  WAVES 

manholes,  were  lifted  from  their  places.  A  block  weighing  7  tons 
in  the  face  of  the  breakwater  at  Ymuiden  was  started  forward  out  of 
its  place  during  a  gale,  the  movement  being  toward  the  waves  which 
were  coming  against  it.  According  to  Gaillard  this  phenomenon 
was  caused  "  by  the  stroke  of  a  wave  compressing  the  air  in  the 
rear  of  it"'*^,  but  similar  results  are  produced  by  expansion  due  to 
the  formation  of  a  partial  vacuum  in  front.  In  1840  a  securely 
fastened  door  in  the  Eddystone  lighthouse  was  burst  outward 
during  the  attack  of  storm  waves,  the  circumstances  leading 
Geikie  to  conclude  that  "  by  the  sudden  sinking  of  a  mass  of 
water  hurled  against  the  building,  a  partial  vacuum  was  formed, 
and  the  air  inside  forced  out  the  door  in  its  efforts  to  restore  the 
equilibrium"^^. 

Another  important  factor  in  the  work  of  waves  is  the  effect  pro- 
duced by  stones,  logs,  blocks  of  ice,  and  other  objects  moving 
with  the  waves.  It  has  been  well  said  by  Playfair^"  that  waves 
thus  armed  become  a  sort  of  "  powerful  artillery  "  with  which 
the  ocean  assails  the  land.  A  large  block  of  ice  or  a  log  may 
concentrate  its  whole  momentum  upon  a  very  small  area  with 
appropriately  great  results.  Thus,  Gaillard  has  suggested  that 
an  exceptionally  high  dynamometer  reading  on  Lake  Michigan 
(when  the  instrument  showed  a  pressure  twice  as  great  as  that 
recorded  in  the  same  locality  for  a  more  severe  storm  and  greater 
than  any  record  for  the  larger  waves  of  Lake  Superior)  may 
possibly  have  been  caused  by  ice  or  timber.  Large  stones  may 
be  hurled  out  of  the  water  with  high  velocities.  At  Tilla- 
mook Rock  on  the  Oregon  Coast,  fragments  of  stones  are  torn 
from  the  cliffs  during  every  severe  storm  and  thrown  on  the 
roof  of  the  light-keeper's  house,  about  100  feet  above  sealevel. 
"  In  December,  1894,  one  fragment  weighing  135  lbs.  was  thrown 
clear  above  this  building,  and  in  falling  broke  a  hole  20  feet 
square  through  the  roof,  practically  wrecking  the  interior  of  the 
building.  Thirteen  panes  of  glass  in  the  lantern  were  broken 
during  the  same  storm  "^^  As  already  noted,  this  lantern  is 
132  feet  above  mean  high  water.  The  fog-signal  siren  horns, 
about  95  feet  above  the  sea,  were  partially  filled  with  rocks  during 
the  storm  of  October  18,  1912.  The  windows  of  the  Dunnet 
Head  lighthouse  on  the  north  coast  of  Scotland,  which  are  over 
300  feet  above  high-water  mark,  are  sometimes  broken  by  stones 
swept  up  the  cliffs  by  waves^^. 


DAMAGE  BY  STORM  WAVES  69 

It  is  perfectly  evident  that  waves  which  are  armed  with 
cobblestones  and  enormous  boulders  must  accomplish  great 
erosive  work  when  they  beat  against  a  cliff  or  artificial  wall.  On 
the  other  hand,  one  must  not  make  the  mistake  of  assuming 
that  waves  which  are  not  thus  armed  can  accompHsh  but  little 
work.  It  is  true  that  large  storm  waves  may  beat  against  a 
cliff  without  removing  the  barnacles  which  are  attached  to  its 
face,  and  that  along  the  shores  of  sahne  lakes  calcareous  tufa 
may  form  on  cliffs  exposed  to  the  impact  of  large  waves^^  But 
this  merely  indicates  that  the  pressure  of  the  liquid  mass  is  so 
evenly  distributed  upon  all  sides  of  the  strong  shell,  or  of  the 
mineral  deposit,  that  the  excess  of  pressure  on  any  one  side  is 
not  sufficiently  great  nor  applied  with  sufficient  suddenness  to 
cause  rupture.  The  same  waves  will  wrench  great  blocks  of 
rock  from  their  places  in  the  cliff  face,  and  drive  air  and  water 
into  joint  crevices  with  such  force  as  to  loosen  large  fragments 
of  the  cliff  and  thus  contribute  to  the  disintegration  of  the  whole 
mass.  A  force  which  exerts  a  pressure  of  thousands  of  pounds 
to  the  square  foot  will  discover  lines  of  weakness  in  any  natural 
cliff.  Even  though  all  sand,  boulders,  and  other  rock  fragments 
were  speedily  carried  out  of  the  zone  of  wave  action,  and  waves 
of  pure  water  alone  attacked  the  coasts,  shorelines  would  retreat 
under  wave  erosion  just  as  surely  as  they  do  when  the  waves 
are  armed  with  abrasive  materials,  although  the  process  would 
certainly  go  on  much  more  slowly. 

British  geologists  have  long  appreciated  the  tremendous  power 
of  the  waves  in  destroying  land  areas,  and  with  good  cause;  for 
no  part  of  the  British  Isles  is  far  removed  from  the  sea,  the  wave 
attack  on  much  of  the  coast  is  remarkably  vigorous,  and  abundant 
ancient  records  and  surveys  permit  careful  computation  of  the 
rate  of  cliff  retreat  at  many  points.  Old  maps  of  Yorkshire  show 
the  location  of  many  towns  and  villages  which  have  been  swept 
out  of  existence  by  the  waves,  their  former  sites  being  now  re- 
presented by  sandbanks  far  out  in  the  sea.  In  1829  there  was 
in  the  harbor  of  Sheringham,  according  to  LyelP'',  a  depth  of  20 
feet  of  water  where  only  forty-eight  years  before  .  had  stood  a 
cliff  fifty  feet  high  with  houses  upon  it.  For  over  half  a  century 
the  chff  at  Happisburgh  retreated  at  the  rate  of  7  feet  per  year, 
while  the  cliff  between  Cromer  and  Mundesley  was  cut  back 
330  feet  in  the  twenty-three  years  previous  to  1861  making  an 


70 


THE  WORK  OF  WAVES 


> 

S2 


O 


DAMAGE  BY  STORM  WAVES  71 

annual  retreat  of  14  feet.  Matthews^^  estimates  that  the  rate 
of  cHff  erosion  on  the  Holderness  coast  of  Yorkshire  varies  from 
7  feet  per  year  in  some  places  to  15  feet  in  others,  while  the 
retreat  between  Cromer  and  Mundesley  since  1861  is  said  to 
have  been  19  feet  annually.  At  South  wold  the  annual  rate  has 
varied  from  15  to  45  feet.  Shakespeare's  cliff  (Plate  VII)  near 
Dover  is  so  vigorously  undermined  that  great  landslides  descend 
from  the  upper  part  of  the  cliff,  the  debris  projecting  far  into  the 
sea  until  the  waves  remove  it  and  renew  their  attack  on  the 
cliff  base.  Such  a  landshde  in  1810  caused  a  marked  earth- 
quake at  Dover.  Detailed  accounts  of  the  rates  of  cliff  erosion 
about  the  British  Isles  will  be  found  in  Lyell's  "  Principles  of 
Geology  "^%  while  Matthew's  "  Coast  Erosion  and  Protection  "^ 
gives  more  recent  data  on  this  question.  Further  details  are 
abundantly  set  forth  in  the  reports  of  the  Royal  Commission  on 
Coast  Erosion  of  Great  Britain^^ 

The  large  blocks  of  rock  dislodged  from  chff  faces,  as  well  as 
smaller  fragments,  are  churned  together  by  the  waves  so  long 
as  they  remain  within  reach,  either  upon  the  beach  slope  or  in 
shallow  water.  The  surf  zone  has  been  Ukened  by  Shaler-^  to 
a  great  mill  in  which  angular  fragments  are  quickly  rounded  and 
everything  in  course  of  time  is  reduced  to  the  size  of  sand  or 
fine  silt  and  swept  out  to  sea.  How  effective  is  this  mill  may 
be  inferred  from  the  fact  that  angular  fragments  of  granite  from 
quarries  on  Cape  Ann,  Massachusetts,  become  fairly  well  rounded 
by  wave  action  in  a  single  year,  while  under  favorable  circum- 
stances "  the  wear  upon  the  pebbles  amounts  on  the  average  to 
several  inches  per  annum  "^°.  On  a  stormy  day  the  roar  of 
grinding  masses  of  boulders  often  rises  above  the  sullen  thunder- 
ing of  the  surf. 

A  vivid  picture  of  the  working  of  the  "  sea  mill,"  which  grinds 
great  boulders  to  sand  and  fine  mud,  is  given  by  Henwood^^  in  an 
account  of  the  visit  made  by  him  to  a  mine  in  southwest  England 
which  extended  out  under  the  sea:  "  When  standing  beneath 
the  base  of  the  cliff,  and  in  that  part  of  the  mine  where  but  nine 
feet  of  rock  stood  between  us  and  the  ocean,  the  heavy  roll  of 
the  larger  boulders,  the  ceaseless  grinding  of  the  pebbles,  the 
fierce  thundering  of  the  billows,  with  the  crackling  and  boiUng 
as  they  rebounded,  placed  a  tempest  in  its  most  appalling  form 
too  vividly  before  me  to  be  ever  forgotten.     More  than  once 


72f  'HE  WORK  OF  WAVES 

doubting  the  protection  of  our  rockj^  shield  we  retreated  in 
affright;  and  it  was  only  after  repeated  trials  that  we  had  con- 
fidence to  pursue  our  investigations." 

Conditions  Affecting  Wave  Energy.  —  We  have  already  seen 
that  the  dimensions  of  waves  vary  with  differences  in  depth  of 
water,  strength  and  duration  of  wind,  and  length  of  fetch  of 
open  water.  It  follows  from  this  that  the  amount  of  wave  energy 
delivered  against  a  shore  will  vary  with  these  same  factors.  A 
coast  bordered  by  off-shore  shallows  escapes  the  most  powerful 
wave  attack,  because  large  waves  cannot  traverse  the  shallow 
water.  Other  things  being  equal,  that  part  of  a  shoreline  facing 
the  greatest  stretch  of  open  water  will  receive  the  largest  amount 
of  wave  energy.  But  it  must  be  remembered  that  the  prevailing 
winds  may  come  across  a  shorter  stretch  of  open  water,  with 
the  result  that  what  appear  to  be  the  less  exposed  parts  of  a 
shore  may  really  suffer  the  more  vigorous  attack.  Account 
must  also  be  taken  of  the  fact  that  the  prevailing  wind  may  not 
be  the  dominant  wind;  for  a  few  great  storms  from  one  direction 
may  more  than  offset  the  effect  of  long-continued  wave  attack 
from  the  direction  of  the  prevailing  wind.  It  is,  therefore,  not 
always  a  simple  matter  to  determine  which  parts  of  a  shore  will 
suffer  most  from  the  energy  of  waves.  The  observer  must 
carefully  consider  the  inclination  of  the  off-shore  slope;  the 
depth  of  water  both  near  the  shore  and  farther  out;  the  pres- 
ence or  absence  of  shallows;  the  directions  of  the  greatest  stretch 
of  open  water,  of  the  prevailing  winds  and  of  the  greatest  storm 
winds;  and  a  number  of  other  factors  which  may  enter  into  the 
case;  and  must  skilfully  weigh  the  relative  importance  of  each 
factor  in  a  given  case  before  he  can  reach  a  safe  conclusion. 

Among  the  factors  affecting  the  energy  with  which  waves 
attack  a  shore  are  two  not  previously  mentioned.  These  are 
tidal  currents  and  the  angle  at  which  the  waves  meet  the  shore- 
line. When  a  wave  encounters  an  opposing  tidal  current,  the 
velocity  and  length  of  the  wave  are  decreased,  the  height  is 
increased,  and  the  wave  may  break  much  as  it  would  on  a  shel- 
ving beach.  A  swiftly  moving  tidal  current  may  thus  be  quite 
as  effective  as  a  shallow  in  causing  large  waves  to  break  before 
reaching  the  shore.  The  south  coast  of  Shetland  is  protected 
from  the  waves  of  a  southwest  storm  so  long  as  a  rapid  tidal 
current  off  the  coast  is  running,  no  matter  how  rough  may  be  the 


CONDITIONS  AFFECTING  WAVE  ENERGY 


73 


O    tn 


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S    ^ 
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T3 


74  THE  WORK  OF  WAVES 

outside  sea;  but  as  soon  as  the  current  ceases  the  surf  breaks  on 
the  shore  witli  great  force^-.  On  the  other  hand,  if  the  current 
is  located  immediately  at  the  shore,  and  especially  if  it  flows  in 
the  direction  of  wave  advance,  the  destructive  power  of  the 
waves  may  be  augmented.  Stevenson  is  of  the  opinion  that 
the  violence  of  the  surf  at  Whalsey  and  Wick  in  northern  Scot- 
land is  in  part  due  to  the  action  of  strong  tidal  currents;  and  at 
certain  other  places  the  surf  seems  to  be  most  destructive  when 
the  tidal  currents  are  strongest''^ 

The  angle  at  which  the  waves  meet  the  shoreline  has  an  im- 
portant effect  upon  the  energy  of  wave  attack.  Waves  are  most 
destructive  when  they  come  in  at  right  angles  to  the  shoreline, 
and  a  very  slight  amount  of  obliquity  materially  decreases  their 
power.  Those  portions  of  the  breakwater  at  Wick  which  are 
assailed  by  waves  coming  "  dead-on  "  have  suffered  much  greater 
damage  than  other  portions  where  the  waves  arrive  at  a  slightly 
oblique  angle^'*.  It  is  therefore  evident  that  where  the  direction 
of  greatest  fetch  of  open  water  makes  an  oblique  angle  with  the 
shoreline,  waves  from  that  direction  may  be  less  destructive  than 
waves  developed  on  a  shorter  stretch  of  open  water  but  approach- 
ing the  land  at  right  angles  to  the  shore.  It  must  not  be  sup- 
posed, however,  that  waves  approaching  a  coast  from  a  given 
direction  maintain  that  direction  until  they  break  upon  the 
beach.  On  the  contrary,  there  is  a  very  marked  tendency  for 
every  wave  to  change  its  direction  in  such  manner  as  to  make  its 
crest  parallel  with,  and  its  direction  of  advance  at  right  angles 
to  the  shoreline.  Inasmuch  as  this  tendency  has  an  important 
effect  upon  the  development  of  shorelines,  we  must  give  it  some 
further  consideration. 

Wave  Refraction.  —  When  a  wave  {ad,  Fig.  12)  advances  toward 
a  coast,  the  direction  of  advance  is  always  at  right  angles  to  the 
wave  crest.  Nearing  the  coast,  the  wave  encounters  shallower 
water  off  the  headlands  than  opposite  the  bays;  and  since  the 
velocity  of  shallow-water  waves  decreases  with  decreasing  depth, 
those  parts  of  a  wave  opposite  headlands  will  lag  behind  the  parts 
opposite  bays,  and  the  wave  crest  will  begin  to  curve  (a^d^)  in 
conformity  with  the  curves  of  the  shorehne.  If  the  headlands 
and  bays  are  not  too  pronounced,  and  if  the  shallowing  of  the 
water  is  not  too  abrupt,  by  the  time  the  wave  has  reached  the 
position  aH^  it  will  have  so  adjusted  itself  as  to  bring  its  crest 


WAVE  REFRACTION 


75 


at  all  points  nearly  parallel  to  the  shoreline.  Or,  as  Harrison^^ 
has  expressed  it,  "  the  velocity  of  the  part  which  first  reaches  the 
shallow  being  lessened,  the  whole  wave  wheels  round,  and 
breaks  nearly  at  right  angles  on  the  beach."  This  process  of 
"  wave  refraction,"  as  Davis  has  called  it,  accounts  for  the  fact 
that  swells  from  distant  storms  ordinaril}^  are  nearly  parallel  to 


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Fig.   12.  —  Diagram   to  illustrate  the  process   of  wave  refraction,   whereby 
wave  attack  is  concentrated  on  headlands.     (After  Davis.) 

the  shore  when  they  break,  no  matter  what  may  have  been  the 
direction  of  the  storm.  Even  within  the  narrow  limits  of  a  single 
curved  beach  an  observer  may  note  the  tendency  of  the  surf  to 
break  directly  on  shore  throughout  its  length,  although  the  beach 
may  describe  an  arc  of  90  degrees  or  more. 

An  important  consequence  of  wave  refraction  is  the  concen- 
tration of  wave  energy  upon  headlands.     Since  the  direction 


76  THE  WORK  OF  WAVES 

of  wave  advance  is  always  at  right  angles  to  the  crestline,  and 
the  latter  becomes  curved  to  conform  with  the  curvature  of  the 
shoreUne,  it  follows  that  a  large  proportion  of  the  waves  will  be 
refracted  toward  the  headlands.  In  Fig.  12  it  is  apparent  that  all 
that  portion  of  the  wave  between  a  and  b  will  be  concentrated 
upon  the  short  stretch  of  shoreline,  AB,  on  the  headland; 
whereas  the  part  of  the  wave  between  h  and  c  will  be  distributed 
over  the  great  stretch  of  the  bay  shore,  BC.  In  other  words, 
wave  refraction  causes  an  enormous  concentration  of  wave  energy 
upon  headlands  and  a  dissipation  of  energy  in  bays.  The  ob- 
server who  wishes  to  witness  the  most  sublime  manifestations  of 
the  power  of  the  sea  must  seek  the  exposed  headlands  of  the 
coast;  while  the  mariner  finds  comparative  safety  within  the 
limits  of  the  bays,  even  where  these  are  broadly  open  to  the  sea. 

Waves  do  not  always  break  parallel  to  the  shore.  In  the 
first  place,  no  wave  can  be  refracted  with  sufficient  abruptness 
to  render  its  crest  parallel  to  the  sharp  and  complex  irregularities 
of  some  shores.  In  the  second  place,  the  water  is  very  deep 
close  to  some  shores,  and  wave  refraction  does  not  begin  to  take 
place  until  the  wave  has  practically  reached  the  headlands. 
The  wave  then  breaks  against  these  projecting  points  of  the 
coast  first,  and  its  remaining  portions,  being  imperfectly  re- 
fracted, sweep  upon  the  shore  from  the  headlands  inward  at  an 
obhque  angle.  Furthermore  "forced  waves,"  or  those  which 
are  still  being  driven  forward  by  the  wind  which  formed  them, 
are  not  so  readily  refracted  as  "  free  waves  "  which  have  passed 
beyond  the  limits  of  the  storm.  It  is  for  this  reason  that  storm 
waves  are  more  apt  to  strike  the  shore  at  an  oblique  angle  than 
are  the  groundswells  which  arrive  during  calm  weather.  The 
more  perfect  refraction  of  the  groundswells  is  due  not  alone  to 
the  absence  of  the  wind's  impelling  force,  but  probably  also 
to  the  fact  that  they  extend  to  greater  depths  and  hence  are 
the  sooner  affected  by  the  refracting  influence  of  a  shallowing 
bottom. 

Depth  of  Wave  Action.  —  The  depth  to  which  the  ocean  waters 
are  affected  by  waves  is  a  matter  of  much  importance  to  all 
students  of  shore  processes.  We  have  already  seen  that 
at  the  depth  of  one  wave  length  below  the  surface  the  water 
particles  of  oscillatory  waves  are  moving  in  orbits  whose  di- 
ameters are  only  5^5  as  great  as  the  diameters  of  the  orbits  at 


DEPTH  OF  WAVE  ACTION  77 

the  surface.  Since  the  period  of  the  lower  orljits  is  identical  with 
that  of  the  larger  surface  orbits,  it  follows  that  the  velocity  of 
the  water  particles  decreases  in  the  same  proportion  as  the  di- 
ameters of  the  orbits.  In  other  words,  the  water  particles  at 
the  depth  of  one  wave  length  below  the  surface  move  with  5^5- 
of  the  velocity  of  the  surface  particles.  The  ability  of  oscil- 
latory waves  to  erode  the  bottom  and  to  transport  debris  there- 
fore diminishes  rapidly  with  increasing  depth,  and  soon  becomes 
negligible.  In  the  case  of  waves  of  translation  the  motion  of  the 
water  particles  is  theoretically  the  same  from  the  surface  to  the 
bottom,  except  that  the  velocity  near  the  bottom  should  be 
somewhat  less  than  near  the  surface,  owing  to  the  fact  that 
the  water  particles  there  pass  through  shorter,  more  nearly  hori- 
zontal paths  in  the  same  length  of  time.  With  these  theo- 
retical points  in  mind  it  will  be  interesting  to  inquire  into  the 
results  obtained  by  different  students  of  this  phase  of  wave 
activity,  and  to  review  their  opinions  as  to  the  maximum  depth 
of  efficient  wave  action  in  nature.  Unfortunately,  few  writers 
distinguish  between  the  effects  of  oscillatory  waves  and  waves 
of  translation. 

Captain  E.  K.  Calver,  R.  N.,  has  observed  waves  which  changed 
their  color  upon  passing  into  water  from  40  to  50  feet  deep  be- 
cause of  their  abrasive  action  upon  the  bottom^^.  Sir  John 
Coode  studied  the  movement  of  shingle  in  the  vicinity  of  the 
Chesil  Bank  on  the  south  coast  of  England,  ])y  descending  to  a 
depth  of  60  or  65  feet  below  the  surface  of  the  sea  in  diving  dress. 
He  found  that  after  a  heavy  storm  the  shingle,  which  was  pre- 
viously covered  with  barnacles,  was  quite  free  from  these  shells, 
proving  a  movement  of  the  coarse  material  at  a  depth  of  nearly 
50  feet^^  According  to  Hermann  Fol,  whose  "  Impressions 
d'un  Scaphandrier  "  are  vividly  recorded  in  the  Revue  Scienti- 
fique  for  1890^^,  a  diver  at  a  depth  of  100  feet  is  tossed  back  and 
forth  by  the  vigorous  oscillatory  movement  of  the  bottom  water 
whenever  groundswells  are  running  on  the  surface.  Hunt  quotes 
the  testimony  of  pilots  and  masters  to  the  effect  that  after  a 
wave  has  broken  over  a  vessel,  sand  is  frequently  left  on  the 
decks  even  when  the  water  has  a  depth  of  75  or  80  feet^^,  and 
describes  a  jar  brought  up  in  a  trawl  from  a  depth  of  220  feet 
into  which  gravel  the  size  of  a  hazelnut  had  been  washed  by 
wave  agitation,     Robert  Stevenson  states  that  fish  disappear 


78 


THE   WORK  OF  WAVES 


DEPTH  OF  WAVE  ACTION  79 

from  the  fishing  grounds  in  the  North  Sea  during  storms,  due 
to  the  agitation  of  the  water  by  wave  action  to  a  depth  of  200 
feet  or  more*°.  The  same  authority  notes  that  at  the  Bell  Rock 
lighthouse,  off  the  east  coast  of  Scotland,  large  stones,  contain- 
ing upwards  of  30  cubic  feet  and  weighing  two  tons  or  more, 
are  often  thrown  upon  the  rock  from  "  deep  water  "  by  the 
waves^^  Thomas  Stevenson  has  made  a  very  interesting  com- 
parison between  the  depths  at  which  mud  reposes  on  the  floor 
of  different  parts  of  the  North  Sea,  and  the  vigor  of  wave  ac- 
tion in  those  places.  He  finds  that  there  is  a  direct  relation 
between  these  two  phenomena,  the  depth  of  the  level  at  which 
mud  accumulates  increasing  in  much  the  same  proportion  as  the 
violence  of  the  waves.  From  this  we  may  infer  that  the  upper 
limit  of  mud  accumulation  is  a  measure  of  the  maximum  depth 
of  wave  disturbance  in  a  given  locahty.  Applying  this  rule 
to  the  North  Sea,  we  find  that  in  protected  areas,  as  the  inner 
parts  of  the  Moray  Firth  and  the  Firth  of  Forth,  and  along  the 
Holland  coast  in  the  narrow  southern  part  of  the  sea,  wave 
action  reaches  to  a  depth  of  25,  50,  or  100  feet;  while  in  exposed 
places  the  disturbance  is  appreciable  to  a  depth  of  from  300  to 
500  feet  or  more^l  According  to  J.  N.  Douglas,  the  fishermen 
off  Land's  End  bring  up  stones  one  pound  in  weight,  which  have 
been  washed  into  their  lobster  pots  at  a  depth  of  180  feet  by  the 
action  of  the  ground-swell,  while  coarse  sand  is  often  washed 
from  a  depth  of  150  feet  by  storm  waves  and  hurled  to  the  lan- 
tern gallery  of  the  Bishop  Rock  lighthouse,  120  feet  above  low- 
water^^  Kinahan  reports  the  moving  of  stones  weighing  several 
hundred  pounds  by  wave  action  in  water  from  90  to  120  feet 
deep  on  the  coast  of  Galway^'*. 

In  contrast  to  the  above  records  of  significant  wave  action 
at  great  depths,  may  be  mentioned  a  few  instances  of  the  ineffi- 
ciency of  wave  action  a  short  distance  below  the  surface.  At 
the  Cherbourg  breakwater  blocks  of  rubble  stone  23  to  26  feet 
below  low-water  are  reported  by  Wheeler  as  remaining  unmoved 
in  the  roughest  sea.  According  to  the  same  authority,  the 
rubble  mound  upon  which  the  Alderney  breakwater  was  later 
erected  remained  three  years  undisturbed  by  winter  storms  below 
the  level  of  15  feet  below  low-water^^  Indeed,  Wheeler  goes  to 
the  extreme  of  limiting  "  the  disturbance  caused  by  the  for- 
mation of  waves  ...  to  a  distance  below  the  surface  about 


80  THE  WORK  OF  WAVES 

equal  to  the  height  of  the  wave"^^  At  Port  Elizabeth,  in  South 
Africa,  Mr.  Shield  found  that  blocks  of  rubble  stone,  weighing 
from  1  to  1|  cwt.  remained  unmoved  at  a  depth  of  22  feet  when 
the  waves  were  15  to  20  feet  high^^  Coode  reports  that  in  the 
same  locality  the  movement  of  sand  on  the  sea-bottom  ceases  20 
feet  below  the  surface^^.  Delesse  states  that  submarine  portions 
of  engineering  structures  are  seldom  disturbed  below  a  depth  of 
16  feet  in  the  Mediterranean,  and  26  feet  in  the  Atlantic^^ 

Too  much  importance  must  not  be  attached  to  the  negative 
results  just  mentioned.     In  some  of  the  cases  we  are  not  in 
possession  of  sufficient  information  regarding  the  degree  of  ex- 
posure of  the  localities  in  question,  or  of  the  size  of  the'  waves 
there  generated.     Engineering  structures  and  masses  of  rubble 
stones  may  be  so  kej^ed  together,  or  may  have  such  external 
forms,  as  to  receive  the  shock  of  vigorous  waves  without  harm, 
while  loose  materials  on  the  bottom  are  at  the  same  time  ma- 
terially affected.     High  waves  of  short  length  will  not  affect  the 
water  to  as  great  a  depth  as  lower  waves  of  greater  length.     The 
positive  evidence  of  wave  disturbance  at  depths  of  several  hun- 
dred feet  is  sufficient  to  prove  that  however  ineffective  some 
waves  may  be,  other  waves  under  favorable  conditions  will  pro- 
duce an  effect  in  deep  water.     We  may  take  600  feet  as  the  limit- 
ing depth  of  ordinary  wave  disturbance,  although  Cornish  sets 
900  feet  as  the  limit  for  the  largest  recorded  waves^".     Geikie 
states  that  ripple  marks  are  sometimes  produced   (by  waves) 
in  fine  sand  at  a  depth  of  600  feet,  and  Airy  apparently  attributes 
the  breaking  of  grounds  wells  in  water  of  the  same  depth  to 
interference    with    the   bottom^^     Agassiz    seems    to    recognize 
the  possibility  of  wave  action  off  the  coast  of  Florida  to  a  depth 
of  600  feet^l     More  definite  figures  are  given  by  Cialdi,  who 
asserts  that  large  waves  will  erode  the  bottom  to  a  depth  of  40 
meters  in  the  English  Channel  and  Adriatic  Sea,  50  meters  in 
the  Mediterranean   Sea,  and  200  meters,    or   about    650   feet, 
in  the  open  ocean;   and  that  at  such  depths  the  waves  will  put 
debris  in  motion  and  grind  it  together^^     Still  more  convincing 
are  the  results  of  experiments  made  by  Siau^^  near  Saint-Gilles 
on  the  Isle  of  Bourbon,  off  the  coast  of  Madagascar.     This  in- 
genious investigator  found  that  by  sounding  with  a  weight  well 
coated  with  tallow  he  could  determine  the  presence  of  ripple 
marks  on  the  sea-bottom  not  only  because  the  impression  of 


DEPTH  OF  WAVE  ACTION  81 

the  ripples  was  imprinted  upon  the  tallow  surface,  but  also  be- 
cause the  heavy  particles  concentrated  in  the  troughs  and  the 
Ught  particles  collected  on  the  crests  of  the  ripples  adhered  to 
the  tallow  in  parallel  bands.  In  this  way  Siau  was  able  to  prove 
the  existence  of  wave-formed  ripple  marks,  and  hence  of  wave 
action,  at  a  depth  of  617  feet.  In  a  letter  to  Nansen^^  Sir  John 
Murray  states  that  great  storms  off  the  north  coast  of  Scotland 
agitate  fine  mud  at  a  depth  of  600  feet.  Murray  also  quotes 
Vionnois  as  authority  for  the  statement  that  in  the  Bay  of  St. 
Jean  de  Luz  the  bottom  is  agitated  during  storms  at  a  depth 
of  300  meters,  or  nearly  1000  feet^^  Unfortunately,  while  these 
authors  evidently  refer  to  oscillatory  waves  in  their  discussions, 
they  do  not  definitely  exclude  the  possibility  that  waves  of 
translation  may  be  responsible  for  the  deep-water  movements. 
Nor  can  we  be  certain,  in  some  of  the  cases  cited,  that  currents 
may  not  have  produced  the  ripple  marks  and  other  phenomena 
attributed  to  wave  action. 

There  is  no  theoretical  reason,  however,  why  we  should  doubt 
the  possibihty  of  appreciable  oscillatory  wave  action  down  to  a 
depth  of  600  feet.  Observations  with  the  naked  eye  and  with  the 
microscope  convinced  the  Weber  brothers  that  during  the  passage 
of  oscillatory  waves  there  is  some  slight  mov  ment  of  the  water 
particles  to  a  depth  below  the  surface  equal  to  350  times  the 
height  of  the  waves^^  Accordingly  a  wave  40  eet  high  should 
affect  water  particles  14,000  feet  below  the  surface.  At  a  depth 
of  but  600  feet  this  movement  must  be  quite  pronounced,  de- 
spite the  rapid  decrease  in  amplitude  of  oscillation  from  the 
surface  downward,  and  notwithstanding  that  the  maximum 
theoretical  depth  of  wave  disturbance  may  not  ordinarily  be 
attained  in  the  ocean  because  of  the  long  time  required  for  the 
downward  transmission  of  surface  oscillations,  which  latter  may 
cease  or  change  direction  before  the  lowest  water  strata  are  set 
in  motion^^  Assuming  a  groundswell  with  a  length  of  1350 
feet  and  a  height  of  16  feet,  which  is  well  within  the  possible 
limits,  the  water  particles  at  a  depth  of  600  feet  (f  of  the  wave 
length)  would  move  in  orbits  having  a  diameter  of  1  foot.  The 
period  of  such  a  wave  is  about  16  seconds;  hence  the  water 
particles  at  the  depth  indicated  would  oscillate  with  a  maximum 
velocity  of  1  foot  in  5  seconds,  or  .06  meter  per  second.  If  at 
the  bottom  the  path  of  oscillation  were  reduced  to  a  straight 


82  THE  WORK  OF  WAVES 

line  1  foot  in  length,  the  velocity  for  a  wave  of  the  same  period 
would  be  about  .04  meter  per  second.  Such  an  oscillation  would 
disturb  clay,  fine  mud,  and  probably  the  very  finest  sands. 
Forbes  has  shown  that  fresh  water  moving  in  a  shallow  trough 
with  a  velocity  of  .077  meter  per  second  will  stir  up  moist  brick 
clay^^,  while  Sorby  recently  found  that  a  current  of  6  inches 
(.15  meters)  per  second  would  drift  along  common  sand  grains 
one  hundredth  of  an  inch  in  diameter  and  that  "  the  very  fine 
Alum-Bay  sand  ^^-q  inch  in  diameter  "  would  be  moved  by  a 
velocity  as  low  as  .04  meter*^".  According  to  de  Lapparent  a 
river  with  a  bottom  velocity  of  .15  meter  per  second  will  trans- 
port coarse  mud^S  whereas  Lyell  says  this  same  velocity  will 
move  fine  sand,  and  Hunt  puts  the  lower  limit  for  ordinary 
fine  sand  at  .10  meter  per  second.  The  foregoing  figures  are 
based  on  observations  in  shallow  fresh  water.  If  we  consider 
the  conditions  of  temperature,  pressure,  salinity  and  viscosity 
which  would  exist  at  a  depth  of  600  feet  in  the  sea,  we  find  that 
sand  particles  of  a  given  diameter  ought  to  be  moved  by  a  slightly 
lower  current  velocity  than  in  the  cases  cited.  It  seems  reason- 
ably certain,  therefore,  that  with  an  orbital  diameter  of  1  foot 
and  a  period  of  16  seconds  there  would  be  appreciable  distur- 
bance of  the  finer  deposits  on  the  sea  floor.  Even  were  the  or- 
bital diameters  as  small  as  1  inch  and  the  period  from  10  to  20 
seconds,  Cornish  is  of  the  opinion  that  the  motion  of  the  water 
would  still  be  sufficient  to  hinder  the  deposition  of  the  finest 
kinds  of  mud''^.  When  associated  with  slow-moving  tidal  or 
other  currents,  a  very  gentle  oscillatory  movement  of  the  water 
due  to  wave  action  may  produce  an  important  effect. 

In  the  words  of  Cornish,  "  We  may  say  with  confidence,  as  a 
theoretical  inference,  that  the  agitation  of  wind-formed  waves 
affects  the  bottom  of  the  sea  as  far  as  the  edge  of  the  continental 
platform  to  such  an  extent  as  (in  co-operation  with  tidal  and 
other  currents)  to  keep  very  fine  mud  moving  about  until  it 
has  an  opportunity  of  subsiding  over  the  edge  of  the  continental 
shelf  "^^  On  the  other  hand,  it  is  evident  that  only  the  finest 
material  will  be  affected  at  such  depths,  and  that  erosion  of  the 
bottom  will  be  almost  imperceptibly  slight,  so  long  as  oscillatory 
waves  alone  disturb  the  water.  Waves  of  translation  are  not  as 
common  in  such  deep  water  as  nearer  shore,  but  whenever  they 
do  occur  we  should  expect,  on  theoretical  grounds,  a  velocity  of 


REFERENCES  83 

the  bottom  water  comparable  to  that  at  the  surface,  and  there- 
fore capable  of  effecting  noteworthy  erosion  and  transportation. 
A  sufficient  body  of  observed  facts  to  estabhsh  this  theory  is  not 
available.  We  are  reasonably  sure,  however,  that  on  exposed 
coasts  the  sea-bottom  is  not  wholly  free  from  some  kind  of  wave 
agitation  down  to  a  depth  of  600  feet  at  least. 

RESUME 

We  have  inquired  into  the  origin  and  character  of  the 
energy  developed  by  waves,  and  have  gained  some  idea  of  the 
tremendous  power  which  they  may  exercise  under  favorable 
conditions.  It  has  been  seen  that  natural  shores,  as  well  as  arti- 
ficial structures,  must  suffer  severely  from  wave  attack.  The 
conditions  affecting  the  vigor  of  wave  action  at  the  shore  have 
briefly  been  discussed,  and  the  effects  of  wave  refraction  con- 
sidered more  fully.  An  inquiry  as  to  the  depth  of  wave  action 
has  resulted  in  the  conclusion  that  the  sea-bottom  is  affected  by 
waves  to  the  edge  of  the  continental  shelf,  or  approximately  to 
a  depth  of  600  feet. 

But  waves  are  not  the  only  forces  of  nature  which  expend 
their  energy  upon  shores.  Currents  of  various  types  play  an 
important  role  in  modelling  shore  forms,  and  must  therefore 
receive  our  attention  before  we  proceed  to  a  study  of  the  evolution 
of  shorelines  under  the  combined  influence  of  waves  and  currents. 


REFERENCES 

1.  Ekman,  V.  W.     On  Dead  Water.     The  Norwegian  North  Polar  Expe- 

dition, 1893-1896,  Scientific  Results.     V,  No.  15,  •..  33,  Christiania, 
1906. 
Flemng,  J.  A.      Waves  and  Ripples  in  Water,  Air,  and  ^Ether,  p.  68, 
London,  1902. 

2.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures. 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  pp.  40, 
46,  Washington,  1904. 

3.  Ibid.,  pp.  40,  45,  47,  50. 

4.  Ibid.,  p.  51. 

5.  Hagen,  G.     Handbuch  der  Wasserbaukunst.      3.   Teil.  Das  Meer.  I. 

Band,  p.  97,  Berlin,  1863. 

6.  Stevenson,    Thomas.      The    Design    and    Construction    of    Harbours. 

3rd  Edition,  p.  98,  Edinburgh,  1886. 

7.  Ibid.,  pp.  61-62. 


84  THE  WORK  OF  WAVES 

8.  Stevenson,  Thomas.  Account  of  Experiments  upon  the  Force  of  the 
Waves  of  the  Atlantic  and  German  Oceans.  Trans.  Roy.  Soc.  Edin. 
XVI,  23-32,  1849. 
[  9.  Gaillard,  D.  D.  Wave  Action  in  Relation  to  Engineering  Structures. 
Corps  of  Engmeers  U.  S.  Army,  Professional  Paper  No.  31,  pp. 
161-171,  Washington,  1904. 

10.  Ibid.,  pp.  164,  167. 

11.  Ibid.,  pp.  194-211. 

12.  Ibid.,  p.  206. 

13.  Stevenson,  Thomas.     Account  of  Experiments  upon  the  Force  of  the 

Waves  of  the  Atlantic  and  German  Oceans.     Trans.  Roy.  Soc.  Edin. 
XVI,  25,  1849. 
Stevenson,    Thomas.     The    Design    and    Construction    of    Harbours. 
3rd  Edition,  pp.  55-57,  Edinburgh,  1886. 

14.  Stevenson,    Thoma  .      The    Design    and   Construction   of   Harbours. 

3rd  Edition,  p.  47,  Edinburgh,  1886. 

15.  Ibid.,  pp.  49-52. 

16.  Meunier,  Stanislas.     La  Geologie  Experimentale,  p.  86,  Paris,  1899. 

17.  Shield,    William.     Principles   and    Practice  of   Harbor  Construction, 

p.  81,  London,  1895. 

18.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures, 

Corps  of  Engineers  U.  S.  Army,   Professional  Paper  No.  31,  p.  126, 
Washington,  1904. 

19.  Geike,  a.     Textbook  of  Geology.     4th  Edition,  I,  568,  London,  1903. 

20.  Playfair,  John.     Ulustratioas  of  the  Huttonian  Theory  of  the  Earth, 

p.  101,  Edinburgh,  1802. 

21.  Gaillard,  D.  D.     Wave  Action  in  Relation  to  Engineering  Structures, 

Corps  of  Engineers  U.  S.  Army,  Professional  Paper  No.  31,  p.   128, 
Washington,  1904. 

22.  Geikie,  a.     Textbook  of  Geology.     4th  Edition,  I,  561,  1903. 

23.  Gilbert,   G.   K.     The  Topographic  Features  of  Lake  Shores.     U.  S. 

Geological  Survey.     5th  Ann.  Report,  p.  81,  1885. 

24.  Lyell,    Charles.     Principles    of    Geology.     11th    Edition,    I,    p.    517, 

New  York,  1873. 

25.  Matthews,   E.   R.      Coast  Erosion    and   Protection,    p.    11,    London. 

1913. 

26.  Lyell,  Charles.     Principles  of  Geology.    Uth  Edition,  I,  671  pp.,  New- 

York,  1873. 

27.  Matthews,  E.  R.     Coast  Erosion  and  Protection.     147  pp.,  London, 

1913. 

28.  Royal  Commission  on  Coast  Erosion.     Minutes  of  Evidence.     Reports 

of  the  Commission.     I,  Part  2,  1-504,  1907. 

29.  Shaler,    N.   S.     Beaches   and  Tidal    Marshes   of  the   Atlantic  Coast. 

National  Geographic  Monograph,  I,  143,  1895. 

30.  Shaler,  N.  S.     The  Geology  of  Cape  Ann,  Massachusetts.     U.  S.  Geo- 

logical Survey,  9th  Ann.  Report,  p.  565,  1889. 

31.  Henwood,    W.   J.     On   the    Metalhferous    Deposits   of   Cornwall   and 

Devon.    Trans.  Geol.  Soc.  Cornwall.     V,  11,  1843. 


REFERENCES  85 

32.  Stevenson,    Thomas.     The    Design    and    Construction    of    Harbours. 

3rd  Edition,  p.  64,  Edinburgh,  1886. 

33.  Ibid.,  pp.  70-72. 

34.  Ibid.,  pp.  35-37,  72. 

35.  Harrison,   J.   T.     Observations  on  the  Causes  that  are  in  Constant 

Operation  Tending  to  Alter  the  Outline  of  the  English  Coast,  to  Affect 
the  Entrances  of  the  Rivers  and  Harbours,  and  to  Form  Shoals  and 
Deeps  in  the  Bed  of  the  Sea.  Min.  Proc.  Inst.  Civ.  Eng.,  VII,  343, 
1848. 

36.  Stevenson,    Thomas.     The    Design    and    Construction    of    Harbours. 

3rd  Edition,  p.  20,  Edinburgh,  1886. 

37.  CooDE,   John.     Description  of  the  Chesil   Bank,   with   Remarks  upon 

its  Origin,  the  Causes  which  have  Contributed  to  its  Formation,  and 
upon  the  Movement  of  Shingle  generally.  Min.  Proc.  Inst.  Civ. 
Eng.,  XII,  534,  1853. 

38.  FoL,    Hermann.     Les    Impressions    d'un   Scaphandrier.     Revue   Scien- 

tifique,  XLV,  715,  1890. 

39.  Hunt,    A.    R.     On    the   Formation    of   Ripplemark.     Proc.    Roy.    Soc. 

London.     XXXIV,  pp.  9,  15,  1882. 

40.  Stevenson,   PlObert.      On  the   Bed  of  the  German  Ocean,   or  North 

Sea.     Memoirs  Wcrnerian  Nat.  Hist.  Soc.  Trans.,  Ill,  332,  1821. 

41.  Ibid.,  p.  332. 

42.  Stevenson,  Thomas..    The  Design  and  Construction  of  Harbours.     3rd 

Edition,  pp.  21-25,  Edinburgh,  1886. 

43.  Douglas,  J.  N.     [On  the  depth  of  wave  action.]     Min.  Proc.  Inst.  Civ. 

Engineers.     XL,  103,  1875. 

44.  KiNAHAN,  G.  H.     The  Travelling  of  Sea  Beaches.     Min.  Proc.  Inst.  Civ. 

Eng.,  LVIII,  284,  1879. 

45.  Wheeler,  W.  H.     A  Practical   Manual   of  Tides  and  Waves,   p.  119, 

London,  1906. 

46.  Ibid.,  p.  118. 

47.  Ibid.,  p.  119. 

48.  CooDE,-  John.     [On  the  depth  of  wave  action.]     Min.  Proc.  Inst.  Civ. 

Eng.,  LXX,  45,  1882. 

49.  Delesse,  M.     Lithologie  des  Mers  de  France,  p.  110,  Paris,  1872. 

50.  Cornish,  Vaughan.      On  Sea  Beaches  and  Sand  Banks.      Geog.  Jour., 

XI,  531,  London,  1898. 

51.  Geikie,  a.     Textbook  of  Geology.     4th  Edition,  I,  562,  London,  1903. 
Airy,  G.  B.     On  Tides  and  Waves.     Encyclopedia  Metropohtana.     V, 

351,  1848. 

52.  Agassiz,  Alexander.    The  Tortugas  and  Florida  Reefs.    Memoirs  Amer 

Acad.  Arts  and  Sciences,  XI,  108,  1888. 

53.  CiALDi,  Alessandro.    Sul  Moto  Ondoso  del  Mare  e  su  le  Correnti  di  csso, 

p.  555,  Rome,  1866. 

54.  SiAu.     De  I'Action  des  Vagues  a  de  Grandes  Profondeurs.     Comptes 

Rendus  de  I'Acadcmie  des  Sciences.     XII,  774-776,  1841. 

55.  Nansen,  Fridtjof.   The  Bathymetrical  Features  of  the  North  Polar  Seas. 

The  Norwegian  North  Polar  Expedition.     IV,  Art.  XIII,  p.  137,  1904. 


86  THE  WORK  OF  WAVES 

56.  Murray,  John.     [On  movement  of  shingle  in  deep  water.]     In  discus- 

sion of  paper  by  John  Coode,  on  Chesil  Bank.     Min.  Proc.  Inst.  Civ. 
Eng.,  XII,  551,  1853. 

57.  Weber,  Ernst  Heinrich  and  Wilhelm.     Wellenlehre  auf  Experimente 

Gegrundet,  p.  126,  Leipzig,  1825. 

58.  Thoulet,  J.     Oceanographie  Dynamique,  p.  57,  Paris,  1896. 

59.  Forbes,  Edward.     Abrading  Power  of  Water  at  Different  Velocities. 

Proc.  Roy.  Soc.  Edinburgh,  III,  474,  1856. 

60.  SoRBY,   H.   C.     On  the  Application  of   Quantitative   Methods   to  the 

Study  of  the  Structure  and  History  of   Rocks.     Quart.  Jour.   Geol. 
Soc.  London.     LXIV,  pp.  180-181,  1908. 

61.  Lapparent,  a.  de.      Traite  de  Geologic;  Phenomenes  Actuels,  p.  183, 

Paris,  1900. 

62.  Cornish,  Vaughan.     Waves  of  the  Sea  and  Other  Water  Waves,  p.  145, 

Chicago,  1911. 

63.  Ibid.,  p.  145. 


CHAPTER   III 
CURRENT   ACTION 

Advance  Summary.  —  Shore  debris  is  subject  to  transpor- 
tation by  many  different  kinds  of  currents.  It  is  the  purpose 
of  the  present  chapter  to  discuss  the  more  important  of  these 
currents  and  to  describe  the  movements  of  debris  which  they 
cause.  Following  a  brief  preliminary  summary  of  the  types  of 
currents  to  be  treated,  there  is  presented  a  detailed  analysis 
of  wave  currents,  and  of  the  profoundly  important  process  of 
"beach  drifting"  for  which  they  are  responsible.  Tidal  cur- 
rents are  next  considered,  and  while  their  value  as  a  factor  in 
beach  construction  has  undoubtedly  been  much  exaggerated, 
it  is  shown  that  they  perform  a  significant  function  in  modi- 
fying shores,  particularly  the  shores  of  estuaries.  Currents  gen- 
(irated  by  seiches  have  but  a  theoretical  importance,  except  in 
a  very  few  localities,  and  therefore  receive  but  scant  attention 
here.  Currents  caused  directly  by  the  friction  of  winds  blowing 
over  water  surfaces  deserve  a  more  extended  treatment.  It 
will  be  seen  that  such  currents  are  in  some  cases  of  a  perma- 
nent character,  in  others  purely  temporary,  while  a  third  group 
varies  in  direction  or  character  with  changes  in  the  seasons. 
Some  of  these  "wind  currents  "  are  far  removed  from  the  lands 
and  consequently  play  no  role  in  shore  development;  but 
others  locally  wash  the  margins  of  continents  or  islands  and  have 
their  share  in  shoreline  work. 

A  special  class  of  currents,  generated  by  winds  but  modified 
by  other  causes,  comprises  the  great  whirls  of  the  major  oceanic 
circulation,  and  these  are  treated  separately  under  the  name 
"planetary  currents."  They  seldom  come  in  direct  contact 
with  the  lands  and  are  therefore  of  minor  importance  to  the 
student  of  shorelines.  Currents  due  to  differences  in  atmos- 
pheric pressure,  and  convection  currents,  are  likewise  shown  to 
play  but  an  insignificant  role  in  shore  processes.  Salinity  cur- 
rents, arising  from  differences  in  specific  gravity  between  Avaters 
having  a  different  salt  content,  are  well  developed  in  certain 

87 


88  CURRENT  ACTION 

straits,  where  they  may  locally  control  the  movements  of  debris. 
For  this  reason  currents  of  this  type  are  treated  somewhat  fully 
and  special  consideration  is  given  to  examples  at  the  mouths  of 
the  Baltic,  Mediterranean  and  Red  seas.  River  currents  and 
their  relation  to  delta  growth  are  briefly  described,  and  a  similar 
treatment  is  accorded  the  "reaction  currents"  which  flow  into 
river  mouths  under  certain  conditions.  Neither  type  of  current 
deserves  a  major  place  in  our  discussion.  Eddy  currents,  fre- 
quently associated  with  some  of  the  currents  mentioned  above, 
deserve  and  receive  a  short  space  in  our  text.  The  important 
hydraulic  currents  generated  as  by-products  of  various  other 
types  of  currents  are  not  treated  separately,  but  in  association 
with  the  movements  with  which  they  are  genetically  connected. 
The  chapter  closes  with  a  special  consideration  of  the  great 
complexities  of  current  action. 

Types  of  Currents.  — 1|  we  define  a  current  as  a  more  or  less 
restricted  body  of  waterVmoving  in  a  definite  direction,  it  is  evi- 
dent that  various  types  of  currents  may  exist  in  the  sea.  During 
oscillatory  wave  motion,  masses  of  water  move  first  forward,  then 
backward ;  and  in  waves  of  translation  there  is  a  forward  movement, 
then  a  halt,  followed  by  another  forward  movement,  and  so  on. 
These  short  but  of  repeated  movements  of  the  water  may  affect 
shore  deposits  in  much  the  same  manner  as  more  continuous 
currents,  and  we  may  therefore  speak  of  them  as  wave  currents. 
They  are  in  many  respects  analogous  to  those  currents  which  are 
associated  with  the  great  oscillatory  movement  of  the  sea  water 
known  as  the  tide,  and  which  are  commonly  called  tidal  currents. 
Bays  and  straits,  as  well  as  lakes  have  periodic  oscillations  of 
their  waters  called  seiches.  If  these  oscillations  are  of  con- 
siderable amplitude,  the  rising  and  falling  of  the  water  give  rise 
to  seiche  currents  which  in  narrow  straits  may  attain  a  fairly 
high  velocity.  It  is  well  known  that  the  wind  tends  to  drag  the 
surface  layers  of  a  water  body  along  with  it,  thus  producing 
within  a  very  short  time  a  distinctly  noticeable  wind  current,  or 
•"  wind  drift  "  as  it  is  more  often  called.  The  great  systems  of 
prevailing  winds  combined  with  the  modifying  effects  of  the 
earth's  rotation,  the  forms  of  land  masses,  and  other  factors,  have 
generated  permanent  systems  of  currents  in  the  principal  oceans. 
These  are  developed  on  a  gigantic  scale  and  are  commonly  dis- 
tinguished from  the  local  currents  produced  by  wind  action  alone. 


TYPES  OF   CURRENTS  89 

Since  currents  of  this  type  must  be  characteristic  of  any  ro- 
tating planet  which  possesses  an  atmosphere  and  oceans,  we  may 
refer  to  them  as  planetary  currents.  Barometric  pressures  being 
greater  in  one  place  than  in  another,  water  may,  under  favorable 
conditions,  flow  from  the  region  of  high  pressure  toward  that  of 
low,  as  more  or  less  distinct  pressure  currents.  If  one  portion 
of  the  ocean  is  more  highly  heated  than  another,  the  difference 
in  density  between  the  lighter  warm  waters  and  the  heavier  cold 
waters  will  give  rise  to  convection  currents  by  means  of  which  the 
waters  will  endeavor  to  re-establish  a  condition  of  equilibrium. 
In  much  the  same  manner  oceanic  waters,  which  are  more  saline 
and  therefore  heavier  than  adjacent  waters,  will  produce  ex- 
change currents  with  the  lighter,  less  saline  waters.  We  may 
for  convenience  call  movements  of  this  origin  salinity  currents. 
When  rivers  enter  the  sea  their  currents  are  progressively  checked 
as  they  proceed  farther  and  farther  into  the  quieter  water;  but 
for  some  distance  out  from  shore  there  may  often  be  recognized 
very  distinct  river  currents.  The  dynamic  force  of  these  out- 
flowing streams  causes  bottom  currents  which  move  landward 
into  the  river  mouths,  and  which  have  been  called  reaction  cur- 
rents. A  current  of  any  origin  may  be  accompanied  by  lateral 
whirls  or  eddies,  and  these  may  be  of  sufficient  diameter  to  give 
eddy  currents  of  considerable  importance.  Whenever  any  one  of 
the  above  types  of  currents  impinges  upon  a  coast,  there  results 
a  piling  up  of  the  water  with  the  consequent  establishment  of  an 
hydrauhc  gradient.  Water  will  flow  from  the  higher  to  the  lower 
level,  and  the  resulting  currents  will  here  be  spoken  of  as  hy- 
draulic currents  ("  polarization  currents  "  of  Cornish^ .  A  valuable 
discussion  of  the  theory  of  some  of  the  above  mentioned  types  of 
currents  will  be  found  in  a  series  of  papers  by  V.  W.  Ekman^ 
published  in  the  "  Annalen  der  Hydrographie  und  Maritimen 
Meteorologie  "  in  1906. 

We  will  now  consider  the  essential  characters  of  the  several 
types  of  currents  in  the  order  named  above,  except  that  it  will 
be  more  convenient  to  treat  the  varieties  of  hydraulic  currents  in 
connection  with  the  original  currents  which  give  rise  to  them. 
We  shall  purposely  omit  consideration  of  currents  which  are  as 
yet  but  little  known,  such  as  the  pulsating  currents  discovered 
off  the  Norwegian  coast  and  described  by  Helland-Hansen^, 
and  the  deep  vortices  in  the  Norwegian  sea  described  by  Helland- 


90  CURRENT  ACTION 

Hansen  and  Nansen*.  We  must  not  forget,  however,  that 
some  of  the  movements  thus  omitted  may  ultimately  prove  of 
importance  to  the  student  of  shoreline  topography,  for,  in  the 
language  of  the  author  last  named,  "  the  sea  in  motion  is  a  far 
more  complex  thing  than  has  hitherto  been  supposed,"  and, 
"  there  must  be  many  forms  of  motion  of  great  and  far-reaching 
importance,  though  hitherto  hardly  known  at  all." 

Wave  Currents.  —  It  has  already  been  shown  that  in  normal 
oscillatory  waves  the  water,  from  the  surface  downward,  moves 
forward  under  the  crest  of  each  wave  and  backward  under  the 
trough.  In  shallow  water  this  alternating  current  movement, 
is  accomplished  without  any  accompanying  vertical  motion  in 
that  part  of  the  water  next  to  the  bottom.  The  forward  and 
the  backward  currents  are  approximately  equal  in  duration,  and 
on  a  level  sea-bottom  should  nearly  compensate  each  other. 
There  will  be  a  slight  advantage  in  favor  of  the  forward  current, 
due  to  the  slight  progressive  motion  of  the  water  particles  in  the 
direction  of  wave  propagation  which  Stokes  has  shown  to  exist 
in  oscillatory  waves^  On  shallow  bottoms  this  would  result 
in  a  slow  advance  of  movable  debris  in  the  direction  of  wave 
propagation. 

Since  the  depth  of  wave  action  depends  mainly  upon  the 
length  of  the  waves,  it  is  evident  that  the  long  groundswells 
which  come  from  distant  storms  will  affect  the  bottom  waters 
more  than  will  shorter  storm  waves  developed  near  the  coast. 
Storm  waves  may  move  landward  or  oceanward,  according  to 
the  wind  direction;  but  the  swells  always  move  landward. 
Hence  the  waves  which  most  affect  the  bottom  come  on-shore; 
and  the  advantage  resulting  from  the  slight  excess  of  the  forward 
component  of  wave  motion  will  be  exerted  mainly  in  a  landward 
direction.  On  a  level  sea-bottom  this  would  mean  a  transpor- 
tation of  movable  debris  prevailingly  in  a  landward  direction. 

The  advantage  just  referred  to  is  often  more  than  offset  by 
the  seaward  slope  of  the  bottom.  If  particles  of  debris  are 
given  a  certain  impulse  up  the  incline,  against  the  pull  of  gravity, 
they  will  travel  a  comparatively  short  distance ;  if  given  a  nearly 
equal  impulse  down  the  incline,  in  the  direction  of  the  pull  of 
gravity,  they  will  move  a  distinctly  longer  distance.  Thus  an 
alternating  current  with  a  slight  excess  of  the  shoreward  com- 
ponent may  cause  a  seaward  transportation  of  debris  on  the 


WAVE   CURRENTS  91 

ordinary  offshore  slope.  Many  authors,  as  for  example  Cor- 
naglia'',  do  not  assign  sufficient  importance  to  the  effects  of 
gravity  on  a  steep  slope,  the  undertow,  and  other  seaward-acting 
components  to  be  discussed  later;  but  consider  that  the  normal 
consequence  of  wave  action  on  the  bottom  is  ordinarily  to  pro- 
duce a  landward  advance  of  debris  of  proper  size  and  specific 
gravity. 

Where  the  water  is  so  shallow  in  comparison  with  the  wave 
length  that  there  is  produced  a  steepening  of  the  wave  front, 
another  element  is  introduced.  As  Cornish'^  has  pointed  out, 
under  these  circumstances  the  forward  motion  is  quick  and  short, 
the  backward  motion  slower  and  of  longer  duration.  This  means 
that  the  shoreward  component  of  such  waves  is  much  the  more 
effective  in  moving  coarser  debris,  since  a  shorter  lived  current 
of  high  velocity  will  transport  material  which  is  too  large  to  be 
moved  at  all  by  the  longer  enduring  but  weaker  seaward  current. 
Sand  and  silt,  on  the  contrary,  will  readily  be  moved  a  nearly 
equal  distance  in  both  directions,  or  on  a  sloping  beach  the  sea- 
ward movement  may  predominate,  as  already  shown.  From  this 
it  follows  that  the  same  waves  may  drive  pebbles  and  cobble- 
stones toward  the  beach  and  finer  debris  toward  deep  water,  at 
one  and  the  same  moment.  Or,  as  Cornish  has  well  expressed  it, 
"  suitable  oscillation  on  a  seaward  slope  will  set  shingle  travelling 
shoreward,  and  sand  simultaneously  travelling  seaward  "^, 

A  further  reason  for  the  landward  progress  of  coarse  debris 
during  wave  action  is  elaborated  by  Cornish  in  his  book  on 
"  Waves  of  the  Sea  and  Other  Water  Waves  "^  He  shows  that 
the  forward  current  begins  just  as  the  vertical  component  of 
wave  motion  is  raising  coarse  material  from  the  bottom,  with 
the  result  that  this  material  is  readily  carried  forward  while  in 
suspension ;  whereas  the  backward  current  sets  in  while  the 
water  particles  are  descending  in  their  orbits  and  are  therefore 
depositing  coarse  material  upon  the  bottom  where  it  is  less 
effectively  moved.  This  argument  loses  much  of  its  force  be- 
cause Cornish  takes  no  account  of  the  fact  that  on  a  smooth 
bottom  the  oscillatory  motion  of  the  water  particles  is  backward 
and  forward  in  a  horizontal  plane,  the  vertical  currents  upon 
which  the  validity  of  his  theory  depends  being  absent.  Im- 
mediately above  the  bottom  the  vertical  element  of  the  oscil- 
lation begins  to  appear,  and  material  carried  upward  a  sufficient 


y. 


WAVE   CURRENTS  93 

distance  by  eddies  due  to  inequalities  of  the  bottom  might  be 
somewliat  affected  in  the  manner  described. 

If  we  turn  our  attention  for  a  moment  to  the  action  of  normal 
waves  of  translation,  we  have  to  note  that  the  currents  which 
they  produce  constitute  essentially  one  intermittent  current 
acting  in  a  uniform  direction.  The  water  particles,  from  the 
surface  to  the  bottom,  move  forward  and  then  stop,  the  process 
being  repeated  with  the  passing  of  every  such  wave.  Accord- 
ingly the  debris  on  the  bottom  is  always  urged  forward;  and 
since  these  waves  usually  come  on-shore,  they  give  rise  to  a 
landward  progress  of  all  movable  material,  both  fine  and  coarse. 
Russell  attributes  the  shoreward  transportation  of  shingle  and 
wreck  to  the  action  of  waves  of  translation^°.  It  appears  cer- 
tain that  either  waves  of  translation  or  oscillatory  waves  may, 
under  proper  conditions,  effect  a  very  remarkable  transport 
of  debris  toward  the  land;  for  Murray"  has  shown  that  shingle 
and  chalk  ballast  dropped  into  the  sea  off  Sunderland  at  a  dis- 
tance of  7  to  10  miles  from  land,  where  the  water  is  from  10 
to  20  fathoms  deep,  are  thrown  on  shore  by  storm  waves;  and 
Gaillard  quotes  Robinson  as  authority  for  the  statement  that  at 
Madras,  during  a  violent  storm,  a  quantity  of  pig  lead,  which 
proved  to  have  come  from  a  vessel  wrecked  more  than  a  mile  off- 
shore^^,  was  cast  upon  the  beach.  The  landward  transport  of  large 
cobblestones  from  deep  water  far  offshore  is  often  effected  "  not 
by  the  simple  impulse  of  the  currents  or  storm  waves,  but  by 
such  action  combined  with  the  buoyancy  given  to  the  stones  by 
the  growth  of  seaweed  attached  to  them  "  (Plate  X),  as  was 
pointed  out  by  Kinahan^^  many  years  ago.  Shaler^^  believes  that 
some  shingle  beaches  receive  their  entire  supply  of  material  in 
this  manner.  In  the  waves  produced  experimentally  by  Caligny, 
which  combined  a  translatory  movement  of  the  water  with  an 
oscillatory  movement,  particles  on  a  level  bottom^  were  trans- 
ported in  a  direction  opposite  to  that  of  the  wave  propagation^^; 
but  we  have  no  sufficient  evidence  that  waves  of  this  type  are 
common  in  nature. 

When  a  wave  breaks  at  the  foot  of  a  beach  slope,  the  water 
which  is  driven  up  the  slope,  forming  the  swash,  carries  material 
landward,  while  the  backwash  tends  to  transport  it  seaward  again. 
The  landward  component  of  this  alternating  current  is  as  a  whole 
the  stronger,  because  the  return  current   suffers   a   loss   of   ve- 


94 


CURRENT  ACTION 


locity  due  to  the  friction  which  acts  continuously,  and  a  loss  of 
volume  due  to  percolation  of  the  water  into  the  crevices  between 
the  sand  and  coarser  material  composing  the  beach.  Where 
the  beach  slope  is  verj-  steep,  however,  the  seaward  current  may 
be  the  more  effective  of  the  two  because  it  works  with  gravity, 
while  the  landward  current  must  propel  the  debris  up  the  slope 
against  the  pull  of  gravity.  It  should  be  noted  that  while  the 
swash  of  the  wave,  advancing  up  the  beach  slope,  may  retain 
something  of  the  forward  component  of  the  true  oscillatory  motion 
belonging  to  the  wave  at  the  moment  of  breaking,  the  backwash 
is  really  an  hydraulic  current  containing  no  element  of  true  wave 
motion. 

Beach  Drifting.  —  If  waves  break  obliquely  upon  a  beach,  there 
results  a  very  important  longshore  transportation  of  debris  on 
the  beach  slope  itself.     To  distinguish  this  phase  of  shore  activity 


Fig.  13.  —  Section  of  beach  slope  showing  by  dotted  Knes  the  so-caUed 
zig-zag  path  of  debris  particles  during  beach  drifting  and  by  soUd  Unes 
the  parabolic  paths  actually  followed. 


from  the  longshore  transportation  effected  by  currents  in  the  water 
just  outside  the  beach,  I  propose  to  call  it  heach  drifting  {"  Strand- 
vertriftung"  of  Kriimmel,  "  Kiistenversetzung  "  of  Philippson). 
Conformable  to  this  usage,  the  longshore  transportation  which 
takes  place  in  the  shallow  water  seaward  from  the  beach,  often 
called  "  longshore  drift,"  will  be  termed  longshore  drifting.  The 
terms  beach  drift  and  longshore  drift  will  then  be  restricted  to  the 
material  transported  by  these  processes,  both  being  included 
under  the  broader  term  shore  drift.  In  the  case  of  beach  drifting, 
the  swash  of  the  wave  advances  obliquely  up  the  slope,  continu- 
ing the  direction  of  advance  of  the  wave;    but  the  backwash, 


WAVE  CURRENTS 


95 


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96  CURRENT  ACTION 

being  under  the  control  of  gravity,  tends  to  return  directly  down 
the  steepest  slope.  As  a  matter  of  fact,  the  control  of  gravity 
replaces  the  oscillatory  movement  of  the  water  gradually  instead 
of  abruptly,  with  the  result  that  the  water  does  not  advance 
in  an  oblique  straight  line  to  the  top  of  the  beach  slope  and  then 
descend  in  a  straight  line  at  right  angles  to  the  shore,  describing 
the  zig-zag  path  shown  by  the  dotted  Knes  in  Figure  13,  but 
rather  describes  a  series  of  parabolic  curves  as  shown  by  the 
sohd  lines.  It  follows  that  a  pebble  under  the  influence  of  such 
wave  action  does  not   strictly  speaking,  pursue  a  zig-zag  course 

along  the  beach  as  is  usually  stated, 
but  rather  a  course  represented  by  par- 
allel parabolas.  Palmer^"  was  the  first, 
so  far  as  I  am  aware,  to  call  attention 
to  the  importance  of  this  phase  of 
wave  activity  in  causing  a  longshore 
transportation  of  debris;  but  his  figure 
illustrating  the  process  of  beach  drift- 
ing incorrectly  represents  a  zig-zag 
path  for  the  transported  material,  and 
contains  a  still  more  serious  error  in 
that  it  represents  the  swash  as  carry- 
ing both  large  and  small  particles  an 
Fig.  14.  — Parabolic  paths  of  ^^^^^^  distance  up  the  beach  slope,  al- 
large  and  smaU  particles  ^^^^^  ^^  recognized  that  small  par- 
of  debris  subject  to  beach     .  ,    °  •    ,    r     ^i  i  .-u 

^1^^-  tides  were   carried   farther   down   the 

slope  by  the  returning  backwash. 
Figure  14  illustrates  the  fact  that  small  particles  describe  bigger 
parabolas  than  larger  debris,  and  therefore  progress  along  the 
beach  with  greater  rapidity. 

The  positions  of  the  paraboUc  paths  taken  by  the  particles  in 
beach  drifting  usually  depend  upon  the  combined  action  of  more 
than  one  set  of  waves,  as  when  the  surf  and  a  superposed  set  of 
wind  waves  strike  the  beach  at  different  angles.  Even  if- the 
surf  breaks  parallel  to  the  beach  there  will  be  some  beach  drifting 
if  the  wind  waves  arrive  at  an  oblique  angle;  but,  as  shown  by 
Figure  15,  the  angle  of  advance  of  the  water  up  the  slope  will  not 
be  as  oblique  as  if  determined  by  the  wind  waves  alone,  since  the 
path  actually  taken  is  the  resuUant  of  the  impulses  given  by  both 
waves.     A  longshore  tidal  current,  or  any  other  current  parallel 


WAVE  CURRENTS  97 

to  and  near  the  shore,  may  combine  with  waves  which  break 
directly  on  shore  to  give  a  very  pronounced  beach  drifting  in  the 
direction  of  the  current.  With  a  longshore  current  moving  in  a 
direction  opposed  to  that  of  the  oblique  waves,  sand  may  travel 


Fig.  15.  —  Parabolic  paths  followed  by  debris  particles  impelled  by  tlie 
combined  action  of  onshore  swells  (broken  lines)  and  obUque  wind 
waves  (solid  lines).     After  Kriimmel. 

with  the  current  and  coarser  material  with  the  beach  drift,  as  was 
fully  recognized  by  Owens  and  Case^^. 

The  direction  of  beach  drifting  will  depend  upon  many  fac- 
tors. Among  these  may  be  noted  the  direction  from  which  the 
groundswells  approach  the  shore,  in  those  cases  where  they  are 
not  sufficiently  refracted  to  strike  the  beach  at  right  angles,  and 
in  which  wind  waves  are  on  the  whole  less  powerful  in  determin- 


98 


CURRENT  ACTION 


M 


O 


WAVE  CURRENTS  99 

ing  the  movement  of  shore  debris.  Another  important  factor  is 
the  direction  of  the  prevaihng  winds,  or  of  the  dominant  storm 
winds,  in  case  these  develop  waves  of  considerable  power,  and 
the  groundswells  are  weak,  or  do  not  approach  the  shore  obliquely. 
The  direction  of  the  greatest  stretch  of  open  water  is  likewise 
important,  since  weak  winds  blowing  over  a  long  stretch  of 
water  may  develop  larger  waves  than  strong  winds  which  cross 
a  limited  water  area.  A  good  example  of  the  effect  of  "length 
of  fetch "  is  found  in  the  beach  drifting  along  the  sandspit 
which  encloses  Toronto  Harbor  on  Lake  Ontario.  Here  the 
movement  of  the  beach  material  is  westward  against  the  prevailing 
westerly  winds,  because  the  greatest  stretch  of  water  over  which 
westerly  winds  can  blow  is  40  miles,  whereas  easterly  winds  cross 
180  miles  of  the  open  lake  surface^^.  Failure  to  recognize  the 
important  relation  of  beach  drifting  to  the  direction  of  greatest 
expanse  of  open  water  has  led  many  authors  to  unsound  con- 
clusions, a  typical  example  being  Haupt's  arguments  against 
the  efficiency  of  beach  drifting  along  the  New  Jersey  and  other 
shores  based  on  the  assumption  that  if  there  were  any  effective 
beach  drifting  it  would  have  to  move  with  the  prevailing  winds^^. 
Haupt  cites  the  well  known  fact  that  on  the  Great  Lakes 
material  may  be  drifted  in  opposite  directions  from  some  point 
near  the  middle  of  one  side  of  a  lake,  and  concludes  this  is  suffi- 
cient proof  that  wind  waves  cannot  be  responsil)le  for  the  move- 
ment. An  inspection  of  Figure  16  will  suffice  to  show  that  this 
conclusion  is  not  justified.  Since  the  dominant  waves  (shown  by 
heavy  lines  in  the  figure)  depend  upon  length  of  fetch  as  well  as 
upon  intensity  and  duration  of  the  wind,  it  is  evident  that  beach 
drifting  between  a  and  c  will  be  southward;  because  the  winds 
from  the  northeast,  blowing  across  a  broad  stretch  of  open 
water,  will  generate  more  powerful  waves  than  the  winds  from 
the  southeast  which  traverse  a  shorter  stretch  of  water,  or  the 
much  more  important  prevailing  winds  from  the  southwest  which 
blow  directly  off  the  land.  Beach  drifting  from  a  to  c  is  thus 
opposed  to  the  chrection  of  the  prevailing  winds.  For  similar 
reasons  the  material  north  of  a  is  drifted  in  the  opposite  direction, 
toward  6;  and  on  the  east  side  of  the  lake  material  is  drifted  in 
opposite  directions  from  d.  The  expectable  directions  of  beach 
drifting  derived  theoretically  in  the  accompanying  diagrams 
(Fig.   16)  appear  to  correspond  with  the  actual   directions   re- 


100 


CURRENT  ACTION 


S^ 


N^ 


Fig.  16.  —  Diagram  to  illustrate  relation  of  l^each  drifting  to  wind  directions 
in  an  ideal  case  and  in  the  case  of  Lake  Michigan.  The  first  two  figures 
show  the  relative  intensities  of  oblique  wave  action  and  the  direction  of 
beach  drifting  on  the  western  and  eastern  sides  respectively  of  an  ideal 
lake  with  winds  blowing  from  all  quarters.  The  third  figure  shows 
reported  direction  of  beach  drifting  along  the  shores  of  Lake  Michigan. 


WAVE  CURRENTS 


101 


ported  for  Lake  Michigan^",  a  lake  somewhat  similar  in  form  to 
the  ideal  lake  of  the  figure  and  similarly  situated  with  reference 
to  the  prevailing  winds.  On  Lakes  Erie  and  Ontario  Wilson^i 
finds  a  similar  relation  between  direction  of  beach  drifting  and 
length  of  fetch  of  open  water.  A  proper  appreciation  of  this 
simple  principle  will  enable  one  to  understand  many  disputed 


Fiu.  17.  —  Parabolic  paths  of  debris  particles  subject  to  beach  drifting  on 
(a)  a  prograding  beach,  (6)  a  retrograding  beach,  and  (c)  a  graded  beach. 

points  regarding  the  movement  of  shore  debris  on  irregular  sea 
coasts. 

Beach  drifting  may  occur  on  a  prograding  beach,  on  a  ret- 
rograding beach,  or  on  a  beach  which  is  at  grade,  i.e.,  one 
which  is  neither  losing  nor  gaining  material.  On  a  prograd- 
ing beach  the  relation  of  slope  to  volume  and  velocity  of  the 
alternating  currents  is  such  that  particles  advance  farther  than 
they  retreat,  and  the  ideal  path  of  a  single  particle  is  represented 
by  Figure  17a.      The  paths  which  particles  on  a  retrograding 


102 


CURRENT   ACTION 


r^ 


Ph 


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o  -2 


O     bJD 


WAVE  CURRENTS  103 

beach  and  on  a  graded  beach  would  tend  to  take  are  shown  at 
b  and  c  of  the  same  figure. 

The  so-called  "  zigzag  "  progression  of  particles  of  debris  is 
commonly  treated  in  connection  with  beach  drifting  only,  as 
though  this  type  of  movement  were  restricted  to  the  zone  of 
breaking  waves  on  the  shore.  Cornaglia^  was  right,  however, 
in  ascribing  such  a  movement  to  debris  on  the  sloping  bottom 
seaward  from  the  beach  during  the  passage  of  unbroken  oscil- 
latory waves  in  a  direction  oblique  to  the  slope.  On  the  bottom 
the  motion  of  the  water  particles,  as  we  have  already  seen,  tends 
to  be  in  a  straight  line,  back  and  forth.  With  waves  oblique 
to  the  slope  this  bottom  movement  (called  "  flutto  di  fondo  " 
by  Cornaglia)  would  carry  the  material  obliquely  up  and  down 
the  slope  over  the  same  path,  with  a  general  advance  or  retreat 
in  the  same  straight  line  when  onshore  or  offshore  components 
prevailed,  were  it  not  for  the  effect  of  gravity.  Under  the  in- 
fluence of  this  force  both  water  particles  and  transported  debris 
tend  to  return  more  directly  down  the  slope  after  each  forward 
oscillation,  with  the  result  that  a  progressive  motion,  parallel 
to  the  shore,  is  added  to  the  back  and  forth  movement.  Or, 
in  common  parlance,  the  particles  "  pursue  a  zigzag  path  " 
(more  properly  a  series  of  parabolic  curves)  on  the  sea-bottom, 
which  results  in  longshore  drifting  of  a  type  analogous  to  beach 
drifting. 

We  shall  find  in  later  chapters  that  many  shore  forms  com- 
monly attributed  to  tidal  and  other  currents  are  more  reasonably 
to  be  interpreted  as  the  product  of  beach  drifting.  The  water 
movements  involved  in  beach  drifting  have  a  high  velocity  and 
hence  a  great  transporting  power;  and  one  may  readily  observe 
coarse  debris  carried  along  the  coast  by  their  force  when  tidal 
and  other  currents  are  too  weak  to  move  anything  but  the 
finest  sands.  Shaler  has  watched  pebbles  made  from  ordinary 
bricks  move  along  the  shore  at  the  rate  of  more  than  half  a 
mile  a  day  under  the  influence  of  beach  drifting,  and  Wheeler-^ 
observed  half  bricks  carried  25  to  30  yards  in  from  1|  to  2  hours 
by  the  same  force. 

Hydraulic  Currents  due  to  Waves.  —  Thus  far  we  have  been  mainly 
concerned  with  those  currents  which  are  more  or  less  directly  in- 
volved in  the  normal  oscillatory  or  translatory  motions  of  the  water 
particles  in  waves.     We  must  now  turn  our  attention  to  the  hy- 


104  CURRENT  ACTION 

draiilic  currents,  which  are  the  indirect  product  of  wave  action.  It 
has  ah-eady  been  shown  that  with  every  wave  of  translation  there 
is  a  direct  shoreward  movement  of  the  water,  which  is  not  com- 
pensated by  a  backward  movement.  Hence  a  series  of  such  waves 
coming  onshore  tend  to  pile  up  the  water  above  the  normal  level 
of  the  sea.  Since  oscillatory  waves  entering  shallow  water  are 
partially  transformed  into  waves  of  translation,  they  too  must 
cause  accumulation  of  wa  er  against  the  coast.  Even  were  they 
not  thus  transformed,  the  slight  excess  of  the  shoreward  com- 
ponent in  waves  of  oscillation  which  has  already  been  described 
would  have  a  tendency  in  the  same  direction.  Thus  on- 
shore waves  raise  the  level  of  the  sea  along  a  coast  upon  which 
they  break.  An  appreciable  local  rise  in  the  sealevel  due  to 
this  cause  has  been  inferred  by  several  writers^^  and  has  been 
demonstrated  by  the  author  at  certain  points  along  the  Atlantic 
Coast. 

It  is  clear  that  the  water  piled  up  against  a  shore  in  the  man- 
ner just  described  must  escape,  thereby  producing  more  or  less 
continuous  "  hydraulic  currents."  If  the  escape  is  seaward, 
along  the  bottom,  we  have  the  current  known  as  the  undertow; 
if  the  escape  is  effected  by  currents  moving  along  the  shore  away 
from  the  area  of  accumulation  in  either  direction,  we  have  a 
longshore  current,  sometimes  called  "longshore  drift."  The 
undertow  may  temporarily  be  checked  under  each  wave  crest, 
and  may  even  have  its  direction  momentarily  reversed  by  the 
forward  moving  water  of  that  part  of  the  wave;  but  under  the 
wave  trough  the  undertow  combines  with  the  backward  moving 
component  of  oscillatory  waves  to  form  a  seaward  bottom  cur- 
rent of  great  strength.  A  marked  development  of  the  under- 
tow is  favored  by  oscillatory  waves,  for  these  disturb  the  bottom 
waters  less  than  the  surface;  by  a  broad  zone  of  waves  striking 
a  long  stretch  of  the  shore  at  right  angles,  since  these  con- 
ditions are  unfavorable  to  the  ready  escape  of  the  water  as 
longshore  currents;  and  by  a  steep  offshore  bottom  and  deep 
water  close  to  shore,  because  the  returning  water  is  then  enabled 
to  pass  quickly  down  beneath  the  disturbed  surface  and  move 
seaward  with  little  interruption.  Longshore  movement  is 
favored  by  waves  of  translation,  since  waves  of  this  class  give 
a  vigorous  shoreward  motion  to  all  the  water  from  the  surface 
to  the  bottom;   by  an  oblique  angle  of  wave  incidence,  because 


WAVE  CURRENTS  105 

water  propelled  obliquely  against  a  shore  tends  to  produce  a 
strong  current  in  the  general  direction  of  the  propulsive  force; 
and  by  gradually  shallowing  water  offshore,  which  favors  the 
development  of  waves  of  translation  and  a  shoreward  movement 
of  the  water  at  all  depths. 

Work  of  Wave  Currents.  —  We  must  conclude  from  what  has 
been  said  in  the  preceding  paragraphs  that  waves  are  profoundly 
important  as  agents  of  erosion  and  transportation,  both  on  shores 
and  shallow  bottoms.  It  is  not  easy  to  understand  the  process  of 
reasoning  which  led  Lieutenant  Davis  to  ignore  the  more  important 
activities  of  wave  currents  in  his  memoir  on  the  various  currents 
of  the  ocean,  and  to  conclude  that  "  the  most  noted  and  interesting 
effect  of  waves  is  the  ripple-mark"^^.  The  careful  reader  of  his 
memoir  will  discover  that  many  of  the  phenomena  ascribed  by 
Davis  to  tidal  action  are  more  probably  the  effects  of  wave  cur- 
rents. In  like  manner  Kinahan^^  reaches  the  conclusion  that  wind 
waves  do  very  little  permanent  work.  He  ascribes  to  tidal 
action  beach  drifting  and  other  phenomena  undoubtedly  pro- 
duced by  wave  action. 

It  is  also  evident  from  the  foregoing  paragraphs  that  the 
action  of  wave  currents  upon  debris  varies  greatly  under  dif- 
ferent conditions.  On  a  flat  bottom  oscillatory  waves  will  move 
debris  prevailingly  shoreward;  but  if  the  slope  be  steep  enough, 
the  same  waves  may  cause  material  to  migrate  seaward;  or 
coarse  debris  may  be  propelled  shoreward  and  fine  debris  sea- 
ward. If  the  waves  belong  to  the  class  of  true  waves  of  trans- 
lation, the  debris  will  be  transported  landward,  even  on  a  sloping 
bottom.  Waves  breaking  on  the  beach  drive  material  up  the 
slope  until  continued  accumulation  makes  the  slope  so  steep 
that  the  backwash  returns  all  material  to  the  breaker  zone.  If 
the  beach  slope  is  too  steep  for  a  given  set  of  waves,  the  back- 
wash will  return  more  material  than  was  brought  by  the  for- 
ward rushing  current,  and  the  beach  will  suffer  erosion.  Beach 
drifting  will  vary  in  direction  and  amount  with  changes  in  the 
direction  and  size  of  the  waves.  The  seaward  component  of 
wave  motion  may  be  effectively  supplemented  by  the  undertow. 
If  the  undertow  is  strong  it  may  prevail  over  the  landward  com- 
ponent of  wave  motion,  and  cause  the  bottom  debris  to  move 
continuously  seaward;  but  if  the  waters  piling  up  against  a 
coast  escape  laterally  as  longshore  currents,   the  debris  may 


106 


CURRENT  ACTION 


first  move  landward,  and  then  suffer  some  longshore  trans- 
portation under  the  influence  of  these  currents.  Since  the  several 
types  of  wave  currents  vary  in  strength  with  the  outline  of  the 
shore,  the  angle  of  offshore  slope,  the  angle  at  which  the  waves 
approach  the  shore,  the  size  of  the  waves,  and  the  kind  of  waves, 
it  is  manifest  that  the  analysis  of  wave  action  upon  shore  debris 
is  no  simple  matter.  This  conclusion  is  amply  justified  by  the 
experience  of  those  engineers  who  have  studied  the  effects  of 
waves  on  natural  shores  and  artificial  structures.  Gaillard^^  ex- 
presses the  general  opinion  of  the  profession  when  he  says  "In 
scarcely  any  branch  of  engineering  are  the  forces  developed 
and  the  methods  and  directions  of  their  apphcation  more  vari- 
able than  in  the  case  of  wave  action."  Before  pursuing  this 
point  further,  let  us  proceed  with  our  inquiry  into  the  behavior 
of  other  types  of  currents. 

Tidal  Currents.  —  The  tides  may  best  be  considered  as  great 
waves  which  combine  some  of  the  features  of  both  oscillatory 


High  Tide 


Fig.  18.  —  Elliptical  orbit  of  water  particle  during  passage  of  the  tide  wave 
over  a  sloping  sea-bottom. 

waves  and  waves  of  translation^^.  They  resemble  oscillatory 
waves  in  having  an  orbital  motion  of  the  water  particles,  the 
orbit  becoming  a  very  much  flattened  elhpse  in  the  shallowing 
water,  with  its  long  axis  rising  toward  the  land  (Fig.  18). 
As  will  appear  from  the  figure,  there  is  a  shoreward  movement 


TIDAL  CURRENTS  107 

of  the  water  particles  until  near  the  time  of  high  tide,  after  which 
a  seaward  movement  takes  place.  These  orbital  movements  of 
the  water  constitute  tidal  currents.  Immediately  at  the  shore 
the  landward  or  "  flood  current  "  may  continue  to  flow  until 
the  very  moment  of  high  tide.  In  the  open  sea,  or  in  the  case 
of  tides  passing  a  headland  projecting  far  out  to  sea,  the  orbital 
path  would  not  be  distorted  as  in  Figure  18,  but  would  be  more 
nearly  circular;  hence  it  is  clear  that  the  landward  movement 
would  persist  for  a  long  time  after  high  tide,  just  as  the  forward 
motion  of  the  water  particles  in  an  oscillatory  wave  continues 
after  the  wave  crest  has  passed  (Fig.  1). 

The  great  importance  of  these  tidal  currents  may  readily  be 
appreciated  from  a  consideration  of  their  velocities.  Kriimmel 
has  shown  that  in  water  30  meters  deep  a  tidal  rise  of  3  meters 
should  result  in  currents  having  a  velocity  of  1.7  knots  per  hour; 
and  with  a  rise  of  4.5  to  6  meters  the  currents  should  attain  a 
velocity  of  over  3  knots  per  hour.  The  observed  velocities  are 
in  agreement  with  the  theoretical  deductions.  According  to 
Wheeler  tidal  currents  in  the  English  Channel  between  Scilly 
and  Hastings  have  a  velocity  of  2  miles  *  an  hour;  in  the  northern 
part  of  the  Wash,  4  miles;  and  off  the  island  of  Ushant,  France, 
6  to  7  knots  per  hour^^.  In  St.  Malo  Bay  where  there  is  a  rise 
of  10  to  12  meters  and  a  water  depth  of  30  meters,  the  velocity 
of  tidal  currents  is  from  5.1  to  6.7  sea  miles  per  hour^".  At 
Hell  Gate  in  New  York  Harbor  the  currents  attain  a  velocity 
of  4.8  knots  per  hour^^  while  Bailey  reports  a  current  of  "  not  less 
than  8  knots  "  through  the  Petite  Passage  southwest  of  Digby 
Gut,  Nova  Scotia^^.  Stevenson  gives  the  velocities  of  a  dozen 
tidal  currents  which  vary  from  a  minimum  of  5.75  to  a  maximum 
of  12.20  statute  miles  per  hour'''.  Sollas  states  that  the  tides 
in  the  Severn  estuary  have  a  velocity  of  from  6  to  12  miles  an 
hour'*,  while  Kriimmel  cites  velocities  of  8  to  10  knots  between 
the  Orkney  and  Shetland  Islands,  11  knots  in  the  dreaded 
"Roost"  of  Pentland  Skerries,  and  11|  knots  in  the  Gulf  of 
Hangchau'^ 

*  It  has  seemed  wisest  to  give  velocities  in  the  units  originally  employed 
by  the  various  authorities,  as  any  attempt  to  convert  the  expressions  into  a 
standard  unit  of  measurement  would  in  some  cases  introduce  a  misleading 
appearance  of  accuracy  if  fractional  parts  of  the  unit  were  employed,  and  in 
other  cases  would  introduce  large  errors  if  the  fractions  were  ignored. 


108  CURRENT  ACTION 

The  transporting  and  eroding  power  of  such  currents  is  enor-. 
mous.  A  velocity  of  but  A  knot  per  hour  will  drive  ordinary 
sand  along  the  bottom,  while  fine  gravel  will  be  moved  if  the 
velocity  rises  to  1  knot;  shingle  about  an  inch  in  diameter  is 
moved  at  2.5  knots;  and  angular  stones  about  one  and  one-half 
inches  in  diameter,  at  3.5  knots^^  Inasmuch  as  tidal  currents 
continue  for  many  miles  in  the  same  direction,  it  is  evident  that 
they  must  play  a  very  important  role  in  the  transportation  of 
shore  debris  and  in  submarine  denudation  whenever  the  velocity 
approaches  the  higher  figures  mentioned  above. 

G.  H.  Kinahan  describes  a  number  of  beaches  and  submarine 
banks  on  the  coast  of  southeast  Ireland  which  he  believes  were 
formed  mainly  by  tidal  currents^^  H.  C.  Kinahan  states  that 
sands  and  gravels  in  "  Beaufort's  Dyke  "  off  the  coast  of  the 
Mull  of  Galloway  are  moved  back  and  forth  by  currents  gen- 
erated by  the  combined  action  of  tides  and  waves  at  a  depth  of 
720  to  860  feet^^.  Along  the  deeper  middle  portion  of  Long 
Island  Sound  the  mean  velocity  of  the  tidal  inflow  is  nearly  1 
meter  per  second  and  of  outflow  slightly  less,  or  high  enough 
to  transport  coarse  gravel.  Dana  shows  that  wherever  there 
is  any  narrowing  of  the  Sound  by  shoals  or  islands  there  is  an 
increase  in  depth,  and  he  attributes  this  to  increased  erosive 
force  of  currents  at  these  points.  He  finds  such  effects  to  a 
depth  of  330  feet^^  In  a  paper  discussing  "  Erosion  durch 
Gezeitenstrome "  Kriimmel  expresses  the  opinion  that  this 
agency  is  responsible  for  the  fact  that  whereas  the  floor  of  the 
Bay  of  Fundy  usually  has  a  depth  of  from  50  to  70  meters  or 
less,  depths  of  from  100  to  110  meters  occur  where  the  -tidal 
currents  are  restricted  by  the  narrows  at  Cape  d'Or  and  Parrs- 
boro^*'.  Reade  ascribes  to  tidal  scour  the  formation  of  trenches 
between  islands  off  the  coast  of  Scotland  having  depths  of 
nearly  800  feet^i;  but  the  possibility  that  these  trenches  repre- 
sent submerged  subaerial  valleys  should  not  be  overlooked. 
The  strong  tidal  currents  of  the  Severn  sweep  along  great  masses 
of  boulders  thereby  deepening  the  channel,  according  to  Sollas; 
and  Richardson  attributes  the  deep  water  known  as  the  "  shoots  " 
to  this  erosive  action''^  Sections  taken  along  the  deep-water 
channel  of  the  Hooghly  River  in  1813  and  1836  showed  that 
between  those  years  tidal  currents  had  scoured  out  the  silt  of 
the  river  bed  to  a  depth  of  52  feet,  forming  a  "  scour  hole  " 


TIDAL  CURRENTS  109 

20,000  feet  long  at  the  top  and  9000  feet  long  at  the  bottom''^ 
Helland-Hansen^^  has  shown  that  marked  tidal  currents  exist 
at  the  bottom  of  fairly  deep  oceanic  waters.  On  the  Michael 
Sars  Expedition,  which  made  the  first  measurements  of  such 
currents  in  deep  water,  he  found  a  true  tidal  movement  of  .27 
meter  per  second  (more  than  .5  knot  per  hour)  at  a  depth  of 
732  meters,  or  2400  feet,  south  of  the  Azores.  KriimmeP^ 
admits  the  efficiency  of  tidal  currents  in  sweeping  rocky  ridges 
free  of  mud  at  a  depth  of  6500  feet  or  more.  When  it  is  re- 
membered that  a  current  of  .20  meter  per  second  or  .4  knot  per 
hour  will  transport  ordinary  sand,  it  is  clear  that  tidal  currents 
may  transport  coast  debris  to  great  depths  and  under  favorable 
conditions  may  even  effect  some  erosion  far  below  the  surface 
of  the  ocean.  Gardiner*"  has  gone  so  far  as  to  attribute  the 
submarine  plateau  of  the  Maldives  to  the  action  of  planetary 
and  tidal  currents  in  cutting  down  a  land  area  to  a  depth  of 
1140  feet  below  sealevel;  but  wh'le  the  theoretical  possibility  of 
such  erosion  must  be  admitted,  the  evidence  on  which  Gardiner 
bases  his  conclusion  in  the  Maldive  case  is  not  convincing. 

Tidal  currents  do  not  always,  or  even  commonly,  act  in  a 
direction  normal  to  the  shoreline.  Along  the  sides  of  a  bay  or 
headland  whose  axis  is  in  the  line  of  tidal  advance,  the  current 
may  be  parallel  to  the  shore.  Shoreline  irregularities  will  de- 
flect the  tidal  waters,  giving  longshore  currents  in  all  possible 
directions.  These  longshore  currents  are  commonly  much 
swifter  than  those  movements  which  take  place  normal  to  the 
beach.  On  an  open  coast,  exposed  to  the  direct  advance  of 
the  tidal  wave,  the  onshore  and  offshore  movements  of  the 
water  are  very  weak,  and  can  accomplish  very  little  geological 
work,  as  can  readily  be  verified  by  the  observer  on  such  a  coast 
during  a  calm  day.  On  the  other  hand,  longshore  currents  close 
to  the  land  are  ver}^  effective  geological  agents,  since  they  remove 
the  debris  produced  by  wave  erosion  and  brought  to  the  sea 
by  rivers,  transport  it  to  distant  localities,  and  often  deposit 
much  of  it  in  deep  water.  They  may  even  produce  profound 
changes  along  the  shores  by  direct  erosion,  as  in  the  case  of  the 
violent  currents  associated  with  the  bore  in  the  estuary  of  the 
Amazon,  the  effects  of  which  have  been  well  described  by  Bran- 
ner''^.  The  currents  which  pass  up  and  down  a  bay  or  estuary 
are  here  considered  longshore  currents;    for  while  they  may  be 


110 


CURRENT  ACTION 


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TIDAL  CURRENTS  HI 

normal  to  the  general  trend  of  the  outer  coast,  they  are  in  general 
parallel  to  the  immediately  adjacent  shores. 

It  is  a  well  known  fact  that  a  narrowing  bay  compresses  a 
tidal  wave  into  smaller  space  and  constrains  it  to  rise  higher. 
Thus  we  get  the  remarkable  tidal  rise  at  the  head  of  the  Bay 
of  Fundy  and  in  the  River  Severn.  It  is  likewise  true  that 
when  the  energy  of  the  tidal  wave  is  transmitted  to  the  smaller 
volume  of  water  in  front,  the  effect  on  the  latter  is  correspondingly 
great.  The  smaller  volume  of  water  develops  a  swifter  current 
and  piles  up  higher  against  the  coast.  If  the  form  of  the  coast 
prevents  the  escape  of  the  accumulated  waters  laterally,  they 
will  continue  to  rise  until  the  head  counterbalances  the  momen- 
tum of  the  advancing  current.  On  the  other  hand,  if  a  large 
bay  is  separated  from  the  open  ocean  by  a  narrow  inlet,  practi- 
cally no  true  tidal  motion  takes  place  within  the  bay.  The  tidal 
wave  is  scarcely  transmitted  through  the  narrow  channel,  and 
the  water  within  the  bay  rises  because  of  the  hydraulic  head 
resulting  from  the  accumulation  of  water  against  the  coast  out- 
side. Such  currents  as  result  from  the  rise  and  fall  of  the  water 
within  the  bay  are  really  hydraulic  currents,  and  are  not  parts 
of  any  true  oscillatory  movement  of  the  water. 

In  bays  and  sounds  the  swiftest  tidal  currents  follow  the 
deepest  channels,  and  are  therefore  not  as  cUrectly  effective  in 
shore  processes  as  when  they  impinge  upon  an  exposed  portion 
of  the  coast.  Even  here,  however,  they  have  an  indirect  effect 
of  no  mean  importance;  for  they  remove  vast  quantities  of 
debris,  which  was  originally  eroded  from  the  land  by  wave  action 
or  carried  to  the  sea  by  rivers,  and  then  transported  by  longshore 
currents  of  different  types  until  brought  within  the  influence  of 
the  inflowing  or  outflowing  tidal  current.  The  inflowing  tide 
sweeps  the  finer  material  far  up  the  bay  where  it  is  deposited 
in  mud  flats  and  tidal  marshes,  which  are  often  reclaimed  for 
cultivation  by  the  process  known  among  the  English  as  "  warp- 
ing "■**,  while  the  coarser  sand  is  moved  landward  a  much  shorter 
distance,  often  forming  bars  along  the  channels.  The  outflowing 
current  carries  the  material  it  receives  out  to  sea,  and  shifts 
the  bars  in  the  same  direction.  Because  of  the  river  water 
usually  poured  into  a  bay  the  ebb  current  predominates  over 
the  flood,  and  the  direction  of  debris  migration  is  prevailingly 
seaward.     The  net  result  of  this  current  action,  therefore,  is  to 


112 


CURRENT  ACTION 


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TIDAL  CURRENTS  113 

favor  wave  erosion  by  removing  debris  to   deep   water,   thus 
keeping  the  shores  better  exposed  to  renewed  attacks. 

Deposition  by  Tidal  Currents.  —  The  great  importance  of  in- 
coming tidal  currents  in  bringing  about  local  deposition  at  the 
heads  of  bays  and  in  the  quiet  waters  of  harbors  justifies  further 
consideration  of  this  point.  There  is  a  tendency  to  ascribe  to 
the  deposited  material  a  fluvial  origin,  and  many  have  argued 
that  only  the  rivers  entering  the  bay  are  capable  of  bringing  so 
much  fine  sediment  to  the  place  of  depositions^.  But  Skertch- 
ley^'^  has  shown  that  the  rapid  silting  up  of  the  Wash  in  eastern 
England,  by  which  a  breadth  of  three  miles  has  been  added  to 
the  land  in  some  places  since  the  Roman  occupation,  is  accom- 
plished by  the  sea  and  not  by  rivers.  Deposition  occurs  mainly 
at  the  slack  of  high  water,  although  a  little  of  the  material  settles 
in  sheltered  places  during  the  ebb.  Crosby^^  ascribes  the  deposi- 
tion of  silt  in  Boston  Harbor  to  the  action  of  incoming  tidal 
currents,  and  shows  that  the  Mystic  River,  which  enters  the 
harbor,  has  effected  scarcely  any  deposition  in  the  lakes  through 
which  it  flows  during  the  same  period  of  time  in  which  a  maxi- 
mum of  25  feet  of  the  silt  has  accumulated  in  the  harbor. 
Mitchelp2  is  of  the  same  opinion  "regarding  detritus  underlying 
salt  marshes  on  other  parts  of  the  New  England  coast.  The 
extensive  deposits  of  red  silt  at  the  head  of  the  Bay  of  Fundy 
are  largely  due  to  the  action  of  the  strong  flood  tide  which  carries 
in  the  material  eroded  from  the  shores  of  the  bay^^.  In  his 
excellent  study  of  the  Severn  estuary  Sollas^*  has  demonstrated 
that  the  flood  tide  not  only  brings  in  large  quantities  of  silt  but 
also  innumerable  remains  of  marine  organisms  which  are  de- 
posited with  the  silt  in  the  upper  reaches  of  the  main  estuary 
and  in  the  tributary  estuaries.  According  to  Browne^^  this  de- 
position takes  place  not  only  at  the  slack  water  of  high  tide, 
but  during  two-thirds  of  the  ebb  tide,  since  he  found  that  in  the 
Avon  below  Bristol  the  silt-laden  lower  waters  remain  stagnant 
long  after  the  surface  waters  have  begun  to  ebb. 

It  should  be  appreciated,  however,  that  even  where  extensive 
deposits  of  silt  are  laid  down  by  tidal  action  at  the  heads  of 
bays,  these  same  tides  transport  much  material  far  out  to  sea, 
where  it  comes  to  rest  in  deep  water.  On  a  subsiding  coast  the 
amount  of  material  deposited  at  the  bay  head  may  exceed  that 
carried  seaward;    and  if  the  subsidence  gives  place  to  stability 


114  '  CURRENT  ACTION 

this  excess  of  deposition  may  continue  for  a  time,  until  the  heads 
of  the  drowned  vallej'^s  are  well  silted  up.  In  time,  a  condition 
of  approximate  equilibrium  will  be  approached,  when  the  swifter 
tidal  currents  of  the  narrowed  channels  will  erode  about  as  much 
material  as  they  deposit.  From  that  time  onward  the  material 
brought  out  by  rivers  and  eroded  from  the  shores  by  waves  will 
add  little  if  anything  to  the  extent  of  the  tidal  deposits.  The 
tidal  currents  charged  with  sediment  will  sweep  up  and  down  the 
bay,  depositing  in  one  place  and  eroding  in  another,  depositing 
at  slack  water  and  eroding  at  times  of  swiftest  flow;  but  each 
retreating  tide,  re-enforced  by  the  outflowing  river  water,  will 
remove  from  the  bay  an  amount  of  material  equivalent  to  that 
brought  in  by  various  agencies.  According  to  Sollas  the  Severn 
estuary  has  reached  this  nicely  balanced  condition,  in  which 
"  the  accumulation  is  always  being  diminished  by  withdrawals 
seaward,  and  as  constantly  renewed  by  fresh  accessions  provided 
b}^  the  denudation  of  the  land  "^^  The  Wash  appears  not  to 
have  reached  this  stage  of  equilibrium,  for,  as  described  by 
Skertchley"  and  observed  by  the  present  writer,  the  deposition 
ia.r  exceeds  the  removal  of  material  and  the  land  gains  upon  the 
sea.  Along  the  head  of  the  Wash  the  average  rate  of  gain  was 
7.29  feet  per  year  from  the  second  to  the  seventeenth  centuries, 
48.65  feet  per  year  during  the  eighteenth  century,  and  31.68  feet 
per  year  during  the  nineteenth  century^*.  It  would  seem  reason- 
able to  suppose  that  with  a  gradual  decrease  in  the  supply  of 
sediment  furnished  to  the  tidal  currents,  an  area  of  tidal  de- 
posits might  pass  bej^ond  the  stage  of  equilibrium  and  enter  a 
stage  in  which  more  material  was  eroded  from  the  region  than 
was  returned  by  the  incoming  tide.  It  is  possible  that  the  head 
of  the  Bay  of  Fundy  has  entered  this  last  stage,  for  in  several 
localities  \nsited  by  me  no  appreciable  accumulation  had  taken 
place  since  the  last  dykes  were  built,  and  in  one  locality  consid- 
erable erosion  had  evidently  occurred,  uncovering  the  ancient 
forest  described  by  Dawson  in  his  Acadian  Geology^^.  Perhaps 
an  excess  of  erosion  over  deposition  is  also  responsible  for  the 
abandonment  of  certain  tide  marsh  areas  and  for  the  increased 
difficulty  of  maintaining  the  dykes;  facts  for  which  Dawson 
suggested  a  change  in  the  direction  of  tidal  currents  as  one  of 
several  possible  explanations^".  A  careful  examination  of  older 
and  later  surveys  of  regions  about  the  head  of  the  bay  might 


TIDAL  CURRENTS  115 

possibly  determine  the  validity  of  the  explanation  here  tenta- 
tively suggested. 

In  the  case  of  a  bay  which  receives  little  or  no  river  water  the 
tidal  regime  may  be  such  that  the  flood  currents  prevail  over  the 
ebb  at  all  times.  Deposition  will  then  exceed  erosion,  not 
merely  until  the  regions  adjacent  to  the  main  channels  are 
silted  up,  but  the  channels  may  themselves  be  blocked  and  the 
tides  completely  excluded  from  the  former  bay  by  their  own 
deposits.  This  fact  led  Browne  to  the  conclusion  that  tidal 
deposition  always  exceeds  tidal  erosion,  and  that  therefore  when 
no  river  water  flows  through  a  tidal  creek  or  bay  to  keep  the 
channels  scoured  out,  such  an  area  must  in  time  silt  up  entirely"^. 
The  arguments  used  to  support  his  conclusion  are  not  convincing, 
and  it  is  probable  that  whether  or  not  deposition  exceeds  erosion 
in  such  a  bay  or  creek  will  depend  on  the  nature  of  the  tidal 
wave  entering  the  depression  and  the  nature  of  the  currents  to 
which  it  gives  rise.  Both  vary  greatly  under  different  conditions, 
and  there  is  no  theoretical  reason,  at  least,  why  the  tidal  regime 
may  not  be  such  as  to  favor  erosion  more  than  deposition  in 
some  cases,  deposition  more  than  erosion  in  others. 

Movement  of  Debris  by  Tidal  Currents.  —  The  seaward  journey 
of  sediment  held  in  suspension  by  tidal  currents  in  an  estuary 
or  tidal  river  is  far  from  simple.  Even  if  we  leave  out  of  con- 
sideration the  shorter  or  longer  halts  made  by  a  given  particle, 
and  imagine  it  to  be  continually  in  transit,  the  ebbing  and  flow- 
ing tides  carry  it  back  and  forth  over  the  same  ground  many 
times,  greatly  prolonging  its  journey.  Because  the  "  land  water  " 
poured  into  the  estuary  by  rivers  causes  the  ebb  tide  to  pre- 
dominate over  the  flood  by  a  greater  or  less  amount,  the  particle 
is  carried  seaward  by  each  ebb  a  Uttle  farther  than  the  following 
flood  carries  it  back;  and  so  it  gradually  makes  its  way  farther 
and  farther  toward  its  final  resting  place  in  deep  water.  An 
exception  to  this  occurs  temporarily  in  some  estuaries  where  the 
resultant  is  landward  while  the  spring  tides  are  strengthening; 
but  this  temporary  upstream  progress  gives  place  to  a  more  pro- 
nounced seaward  advance  after  spring  tides  are  past.  Numerous 
experiments  with  nearly  submerged  floats  have  demonstrated 
the  predominance  of  the  seaward  component  in  such  tidal  oscilla- 
tions. Figure  19  shows  the  course  taken  by  such  a  float  in  New 
York  Harbor,  where  the  Metropolitan  Sewerage  Commission  has 


116 


CURRENT  ACTION 


L  o  iv^e  r      Bay 


Finish 


Fig.  19.  —  Course  followed  by  a  nearlj^  submerged  float  under  the  influence 
of  tidal  currents  in  New  York  Harbor.     (After  Parsons.) 


TIDAL  CURRENTS 


117 


studied  the  effect  of  tidal  currents  on  the  transportation  of 
sewage.  Figure  20  shows  the  theoretical  path  during  successive 
tides,  which  a  particle  would  take  on  this  same  journey,  and  it 
will  be  seen  that  the  theoretical  and  actual  paths  agree  closely. 
Both  demonstrate  the  seaward  migration  of  particles  in  suspen- 
sion. The  comparative  volumes  of  the  ebb  and  flood  currents, 
responsible  for  this  seaward  migration,  may  be  seen  in  the  follow- 
ing table  taken  from  a  paper  by  Parsons^-.  The  importance  of 
river  water  in  augmenting  the  ebb  is  clearly  apparent  from  this 
table. 


VOLUMES    FLOWING    OX    EBB    AND    FLOOD    CURRENTS, 
HARBOR  OF   NEW   YORK,    IX   MILLIOXS   OF 
CUBIC   FEET 


Part  of  harbor 

Yearly  means 

Ebb 

Flood 

12,041 
7,430 
6,990 
6,230 
3,980 

10,779 

Hnrl'^nn  River    off  the  Batterv    

6,343 

"             '<         "  39th  Street 

5,903 

"             "        "    Fort  Washington  Point 

"            "        "   Tarry  town      

5,143 
2,893 

In  branches  of  the  harbor  where  little  land  water  enters,  the 
difference  between  ebb  and  flood  volumes  is  not  so  great.  In 
other  harbors  where  larger  rivers  than  the  Hudson  enter,  the 
difference  must  be  much  more  marked.  Experiments  with 
floats  in  the  Thames  estuary  show  that  the  average  seaward 
progression  of  a  particle  in  suspension  is  |  mile  a  day^^ 

One  result  of  the  back-and-forth  journeying  of  each  particle 
is  the  accumulation  in  estuaries  and  tidal  rivers  of  a  vastly 
greater  amount  of  material  than  is  daily  contributed  to  their 
waters.  "  Thus  in  the  waters  of  the  Severn  estuary  there  is  a 
storage  of  suspended  sediment,  the  accumulation  of  as  many 
days,  or  weeks,  or  months  as  are  occupied  in  its  wanderings  to 
and  fro  "'\ 

There  may  be  some  question  as  to  whether  the  coarser  material 
on  the  bottom  of  estuaries  and  tidal  rivers  always  has  a  tendency 
to  move  prevailingly  seaward.  Such  material  is  certainly 
shifted  back  and  forth  by  the  ebb  and  flood  currents,  as  shown 


118 


CURRENT   ACTION 


Mt.  St. Vincent 


Grants  Tomb 


in  the  case  of  a  sunken  vessel  at  the  mouth  of  the  Gironde  which 

had  the  sand  scoured 
from  about  it  by  the 
ebb  current  and  was 
completely  buried 
again  by  the  flood 
current^^  Experi- 
ments with  floats 
show  the  predomi- 
nance of  the  seaward 
component  of  tidal 
oscillations  at  or  near 
the  surface  of  the 
w^ater,  but  do  not  tell 
us  about  the  move- 
ments in  depth.  It 
seems  probable,  how- 
ever, that  there  is 
ordinarily  a  similar 
seaward  tendency  in 
the  deeper  waters  also. 
The  seaward  compo- 
nent of  the  oscillation 
has  a  further  advan- 
tage in  that  it  w^orks 
with  gravity ;  for  par- 
ticles of  sand  will 
travel  farther  down 
the  slope  of  the  chan- 
nel with  the  ebb  than 
they  will  up  the  slope 
with  a  flood  current 
of  equal  velocity  and 
duration.  But  three 
"factors  at  least  tend 
to  give  an  advantage 
to  the  flood  current 
in  certain  cases.  The 
waters  of   a  bay  are 


The  Battery 


The  Narrows 


Sandy  Hook  Beacon 


Scotland  Lightship 


Fig.  20.  —  Theoretical  course  calculated  by  Par- 
sons for  the  float  whose  actual  course  is 
shown  in  Figure  19. 


often  much  fresher  than  those  of  the  ocean,  because  of  dilution  by 


TIDAL  CURRENTS  119 

rivers.  Heavier  salt  water  from  the  ocean  may  therefore  push  in 
along  the  bottom  while  the  surface  waters  are  still  flowing  sea- 
ward'^''. This  action  will  be  facilitated  by  differences  in  tempera- 
ture, if  the  waters  of  the  bay  are  warmer  than  those  of  the  ocean". 
Mitchell  has  shown  that  at  the  mouth  of  the  Hudson  River  the 
bottom  water  moves  landward  with  a  velocity  of  .6  meter  per 
second,  or  more  than  1  knot  per  hour,  while  the  surface  waters 
are  still  ebbing*'^  During  part  of  the  ebb  tide,  therefore,  trans- 
portation of  bottom  debris  may  \ye  landward.  Furthermore, 
since  Browne^*  has  shown  that  the  bottom  waters  may  also  be 
stagnant  during  part  of  the  ebb,  it  would  seem  possible  to  have 
a  case  in  which  the  flood  waters  entered  a  bay  along  the  bottom 
during  the  last  of  one  ebb  tide,  then  remained  stagnant  while 
the  surface  flowed  back  to  the  sea  during  much  of  the  following 
ebb.  If  flood  currents  thus  predominate  on  the  bottom  and  ebb 
currents  at  the  surface,  fcoarser  material  will  migrate  landward 
while  material  in  suspension  is  carried  seaward.  Mitchell  was 
of  the  opinion  that  the  flood  does  predominate  in  New  York 
Harbor  below  a  depth  of  6  fathoms^",  and  that  it  would  cause 
the  bar  at  the  harbor  mouth  to  advance  up  the  channel,  were 
this  action  not  prevented  by  other  currents".  As  the  coarser 
material  of  a  landward  moving  deposit  is  ground  to  a  finer  size, 
however,  it  will  rise  in  suspension  and  move  seaward  toward  the 
ultimate  goal  of  all  land  debris  —  quiet,  deep  water. 

A  second  factor  favoring  the  landward  movement  of  bottom 
debris  arises  from  the  change  in  form  which  the  tidal  wave  some- 
times experiences  when  entering  a  bay  or  tidal  river.  The  front 
of  the  wave  becomes  steeper  than  the  back,  and  the  flood  current 
is  much  stronger  than  the  ebb,  the  latter  lasting  a  longer  time. 
An  extreme  case  of  this  inequality  of  current  velocity  is  found 
in  the  tidal  "bore"  or  ''eager"  which  invades  certain  rivers, 
notably  the  Tsien-Tang-Kiang  of  China,  where  the  front  of  the 
wave  sometimes  appears  as  a  wall  25  feet  high,  and  a  million 
and  a  quarter  tons  of  water  may  be  carried  by  a  given  point  in 
one  minute^2  The  vigorous  current  of  the  "  pororoca,"  or  bore 
of  the  Amazon  River,  has  already  been  mentioned^^.  Under 
such  conditions  bottom  debris  which  is  entirely  too  coarse  to  be 
affected  by  the  longer  continued  but  weaker  ebb  current  may  be 
carried  forward  by  the  flood.  It  would  seem,  therefore,  that 
conditions  may  exist  which  compel  a  landward  migration  of 


120  CURRENT   ACTION 

bottom  debris  under  the  influence  of  tidal  currents,  although  this 
material  when  ground  finer  would  move  seaward  under  the  same 
tidal  regime.  A  sufficient  body  of  observations  is  not  yet  avail- 
able to  enable  one  to  determine  how  widespread  these  conditions 

may  be. 

Where  beach  deposits  above  mean  sealevel  are  subject  to 
transportation  by  tidal  currents,  either  with  or  without  the  aid 
of  wave  currents,  there  may  be  a  marked  tendency  for  such  shore 
debris  to  migrate  in  the  direction  of  the  flood  current.  This 
arises  from  the  fact  that  where  the  tide  flows  freely,  high  water 
coincides  more  or  less  closely  with  the  flood  and  low  water  with 
the  ebb.  Hence  those  beach  deposits  above  mean  sealevel  will 
be  moved  by  the  flood  current,  but  will  not  be  reached  by  the 
ebb  current.  In  bays  and  inlets  this  striking  difference  in  the 
efficiency  of  flood  and  ebb  currents  in  transporting  beach  material 
is  less  marked  than  opposite  headlands,  because  near  the  bay 
heads  flood  begins  when  the  tide  is  lower  and  ebb  commences 
soon  after  high  water  is  attained.  It  follows,  therefore,  that  the 
debris  which  migrates  along  the  shore  from  the  headlands  toward 
the  bay  heads  under  control  of  the  dominant  flood  must  come  to 
rest  where  flood  and  ebb  more  nearly  neutrahze  each  other. 
This  should  give  rise,  in  the  absence  of  counteracting  influences, 
to  a  tidal  accumulation  of  debris  in  the  heads  of  bays  and 
inlets^*. 

In  a  bay  which  has  no  strong  tidal  currents,  the  incoming 
flood  may  be  incapable  of  stirring  up  any  appreciable  quantity 
of  sediment,  so  that  Uttle  material  is  carried  to  the  bay  heads 
for  deposition.  On  the  other  hand,  the  ebb  tide  augmented  by 
the  outflow  of  river  water  may  be  sufficiently  strong  to  carry 
into  deeper  water  such  sediment  as  is  brought  in  by  the  rivers 
or  suppHed  by  wave  erosion.  Under  these  conditions  tidal 
deposits  at  the  bay  heads  mil  be  conspicuous  by  their  absence. 
The  small  amount  of  such  deposits  at  the  mouths  of  rivers  enter- 
ing Chesapeake  Bay  may  perhaps  be  thus  explained. 

Along  very  irregular  shores  the  comparative  strength  of  flood 
and  ebb  currents  can  scarcely  be  predicted.  Each  area  must 
be  studied  for  itself.  The  positions  of  channels  between  islands 
and  shoals,  with  reference  to  the  direction  of  advance  of  the 
currents,  may  be  such  that  some  channels  will  have  strong 
flood  currents  and  very  little  ebb,  while  others  will  have  vigorous 


TIDAL  CURRENTS  121 

ebb  currents  and  scarcely  any  movement  during  the  flood. 
Bache"  found  the  ebb"  currents  near  Sandy  Hook  much  more 
powerful  than  the  flood,  and  was  indeed  of  the  opinion  that  ebb 
currents  are  practically  always  iax  more  important  than  the 
flood  as  eroding  and  transporting  agents.  The  process  of  reason- 
ing by  which  he  reaches  this  conclusion  does  not  seem  convincing; 
and  while  the  predominance  of  ebb  currents  in  bays  receiving 
upland  waters  may  be  admitted  as  a  general  rule,  subject  to 
certain  exceptions,  the  relative  strength  of  flood  and  ebb  in  the 
straits  and  other  channels  of  an  irregular  coast  must  be  more 
variable. 

Hydraulic  Currents  Due  to  Tides.  —  The  changes  in  surface  level 
of  the  ocean  resulting  from  tidal  action  inevitably  cause  the  for- 
mation of  various  types  of  hydraulic  currents,  which  we  may  now 
briefly  consider.  When  the  tide  rises  higher  on  one  part  of  the 
coast  than  on  another,  any  part  of  the  water  which  does  not  partici- 
pate fully  in  the  tidal  osciflation  will  flow  from  the  higher  toward 
the  lower  level  under  the  influence  of  gravity.  We  may  thus  get 
hydraulic  currents  having  the  same  periodicity  as  true  tidal  cur- 
rents''^  The  waters  piled  up  against  a  coast  by  a  rising  tide  may 
escape  to  either  side  as  longshore  hydraulic  currents;  or  if  the 
waters  are  piled  up  at  the  head  of  a  converging  bay  so  that  lateral 
escape  is  not  possible,  there  may  be  developed  an  undertow  which 
will  give  a  seaward  motion  to  the  bottom  waters  before  the  di- 
rection of  the  surface  current  is  reversed'''^. 

In  the  case  of  a  bay  separated  by  a  narrow  inlet  from  the  open 
sea,  the  tide  in  the  ocean  rises  so  rapidly  that  enough  water  can- 
not pass  through  the  inlet  to  keep  the  bay  surface  rising  at  the 
same  rate.  Later  the  tide  in  the  ocean  will  fall  more  rapidly 
than  the  surface  of  the  bay,  because  the  outflowing  water  es- 
capes through  the  narrow  inlet  so  slowly.  Consequently  the 
ocean  surface  is  highest  part  of  the  time,  while  the  bay  surface 
is  highest  at  other  times.  These  differences  of  levels,  which 
may  amount  to  a  number  of  feet  where  the  tidal  range  is  large, 
give  rise  to  hydraulic  currents  into  and  out  of  the  bay.  Such 
currents  may  have  a  very  steep  gradient  and  correspondingly 
high  velocity,  as  in  the  case  of  those  at  the  narrow  entrance  to 
St.  John's  Harbor,  New  Brunswick,  where  the  average  maximum 
head  is  nearly  10  feet,  and  a  reversible  fall  is  produced,  facing 
inward  when  the  water  in  the  ocean  is  highest,  and  outward 


122  CURRENT  ACTION 

when  the  water  in  the  harbor  is  highest.  HydrauHc  currents  of 
this  type  are  important  features  at  the  inlets  connecting  the 
ocean  with  lagoons  behind  offshore  bars  along  much  of  the 
Atlantic  coast. 

HydrauHc  currents  greatly  complicate  the  true  tidal  move- 
ments of  coastal  waters.  An  idea  of  their  importance  may  be 
gained  from  an  inspection  of  the  review  of  tidal  currents  for 
different  parts  of  the  world  given  by  Harris  in  his  "  Manual  of 
Tides,"  where  many  of  the  associated  hydraulic  currents  are 
mentioned''^.  According  to  Parsons  the  tidal  currents  in  New 
York  Harbor  vary  greatly  in  character,  some  being  almost 
wholly  oscillatory,  others  almost  wholly  hydraulic,  and  the  re- 
mainder combining  both  elements  in  varying  proportions^^ 
Many,  if  not  most,  of  the  tidal  currents  observed  along  a  coast 
are  compound  currents,  consisting  in  part  of  true  oscillatory 
movements  of  the  water  and'  in  part  of  hydraulic  movements. 
This  is  doubtless  true  of  many  of  the  tidal  currents  whose  velo- 
cities are  noted  on  a  preceding  page. 

Seiche  Currents.  —  The  phenomena  of  seiches  have  already 
been  described.  It  is  evident  that  the  rising  and  falling  of 
water  due  to  seiches  in  a  lake,  or  in  a  bay  of  the  ocean,  must 
produce  currents.  As  a  rule  these  currents  are  so  feeble  in  the 
main  water  body  as  to  be  scarcely  perceptible;  but  if  the  waters 
are  compressed  into  a  narrower  or  shallower  space,  they  may 
acquire  an  appreciable  velocity.  If  the  waters  temporarily 
raised  or  lowered  at  one  end  of  the  basin  are  connected  by  a 
narrow  strait  with  another  water  body,  hydraulic  currents  of 
considerable  force  may  be  produced  in  the  strait.  The  remark- 
able currents  in  the  Strait  of  Euripus*"  appear  to  be  largely  of 
this  origin.  According  to  tradition  Aristotle  plunged  into  these 
turbulent  waters  in  despair  because  he  could  not  solve  the 
mystery  of  their  movements.  The  behavior  of  the  water  in  the 
Strait  is  enough  to  justify  the  tradition,  for  the  seiche  currents 
are  combined  with  tidal  currents  in  such  manner  as  to  give  nearly 
normal  tidal  movements  for  several  days,  followed  by  another 
period  in  which  the  waters  ebb  and  flow  twelve  or  fourteen 
times  a  day^^  "  The  currents  .  .  .  are  so  violent  that  mills  are 
kept  in  operation  by  them  "^2.  Seiche  currents  must  frequently 
modify  tidal  and  other  currents  to  an  extent  not  yet  determined; 
but  it  does  not  seem  probable  that  they  are  often  so  strongly 


WIND  CURRENTS  123 

developed  as  naterially  to  affect  shoreline  processes.  Even  in 
the  Strait  of  Euripus,  Cold^''  was  unable  to  find  any  effect  of  the 
seiche  currents  upon  the  shores. 

Wind  Currents.  —  When  wind  blows  over  water  it  tends  to 
drag  the  surface  particles  of  the  water  along  with  it.  Thus 
the  water  surface  acquires  a  motion  in  the  direction  of  the  wind, 
although  the  velocity  of  the  water  never  equals  that  of  the  wind. 
Because  of  the  viscosity  of  water  this  motion  is  gradually  com- 
municated to  the  deeper  layers  but  with  rapidly  diminishing 
intensity.  It  has  been  demonstrated  that  in  course  of  time 
continuous  wind  action  upon  an  unconfined  ocean  would  set 
the  entire  body  of  the  ocean  in  motion*^.  The  surface  currents 
produced  by  wind  are  often  spoken  of  as  wind  drift,  or  drift 
currents;  but  since  the  term  "  drift  "  is  also  applied  to  currents 
of  almost  any  origin  which  happen  to  flow  parallel  to  the  coast, 
as  well  as  to  shore  detritus  which  is  being  moved  by  such  cur- 
rents, and  since  the  terms  "drift"  and  "drifting"  are  used  in  a 
restricted  sense  in  this  volume,  it  will  be  better  for  sake  of 
clearness  to  employ  the  term  "  wind  currents  "  when  referring 
to  the  currents  now  under  discussion.  This  term  is  not  wholly 
satisfactory,  as  it  suggests  rather  too  strongly  the  air  currents 
which  are  the  cause  of  the  water  currents  here  considered;  but 
since  air  currents  cannot  properly  be  called  "  wind  currents,"  and 
since  the  term  wind  currents  is  analogous  to  the  terms  wave 
currents,  tidal  currents,  and  seiche  currents  already  used,  we 
may  continue  to  speak  cf  wind  currents  until  a  better  term  is 
suggested. 

The  velocity  of  wind  currents  will  depend  upon  the  strength 
of  the  wind,  the  length  of  time  it  has  been  blowing,  and  the 
size  and  shape  of  the  water  body.  In  the  open  ocean  the  sur- 
face waters  under  the  trade  winds  ordinarily  have  a  velocity  of 
from  15  to  25  miles  per  day.  Along  the  shore  a  velocity  of 
three  or  four  miles  an  hour  during  a  strong  wind  is  not  unknown. 
Harrington^^  reports  wind  currents  on  the  Great  Lakes  moving 
from  2  to  3  miles  per  hour,  and  Taylor^^  observed  a  current  on 
the  east  shore  of  Lake  Michigan  which  moved  northward  under 
the  influence  of  "  a  strong  sou'wester  "  with  an  estimated 
velocity  of  4  miles.  Currents  of  such  velocities  moving  over  a 
shallow  bottom  parallel  to  the  shore  are  doubtless  effective  in 
the  longshore  transportation  of  debris,  helping  to  remove  eroded 


124  CURRENT  ACTION 

material  from  the  bases  of  cliffs  and  river-brought  sediment  from 
opposite  stream  mouths,  as  well  as  determining  the  character 
of  the  shores  where  their  loads  are  deposited. 

Hydraulic  Currents  Due  to  Winds.  —  Wind  currents  are  ex- 
tremely effective  in  causing  hydraulic  currents.  If  a  wind  cur- 
rent impinges  directly  upon  a  coast,  the  water  is  piled  up  above 
its  natural  level.  In  shallow  water  bodies,  or  on  a  shelving  shore, 
the  rise  in  level  may  be  very  marked,  but  is  slight  on  steep  coasts 
with  deep  water  close  in  shore.  The  heaped  up  waters  must 
escape  to  one  side  or  along  the  bottom.  If  the  latter  mode  of 
escape  prevails,  the  seaward  moving  waters  resemble  the  under- 
tow, and  may  assist  in  the  removal  of  fine  debris  to  deep  water. 
Such  currents  are  sometimes  spoken  of  as  "  counter  currents  in 
depth."  Southeasterly  winds  in  summer  drive  the  surface  waters 
of  the  Gulf  of  Cahfornia  northward  toward  the  head  of  the  Gulf, 
whence  there  is  no  opportunity  for  escape  on  the  surface.  At  a 
depth  of  50  meters  the  water  is  found  to  be  moving  southward^^ 
Hunt  observed  a  strong  bottom  current  flowing  out  of  Torquay 
Harbor  when  a  gale  drove  the  surface  water  inward^^. 

The  reverse  of  this  circulation  occurs  when  winds  blow  off- 
shore, driving  the  surface  waters  out  to  sea.  Bottom  currents 
then  move  in  toward  the  land  to  replace  the  water  driven  away 
by  the  wind.  On  the  coast  of  Europe  bathers  are  familiar  with 
the  fact  that  the  water  is  warmer  when  the  winds  blow  toward 
the  land  and  colder  when  they  blow  in  the  opposite  direction. 
This  is  because  onshore  winds  pile  the  warm  surface  waters 
up  against  the  coast,  and  the  colder  bottom  water  escapes  sea- 
ward; while  offshore  winds  blow  the  warm  water  away  from  the 
coast,  and  colder  bottom  water  moves  in  to  take  its  place.  The 
northeast  trade  winds  continually  blow  the  surface  water  away 
from  the  northwest  coast  of  Africa,  with  the  result  that  abnor- 
mally cold  water  is  always  found  near  that  shore,  having  moved 
in  from  the  offshore  depths^^.  Similar  effects  are  produced  in 
winter  on  the  northeast  coast  of  North  America  by  the  prevail- 
ing westerhes^",  and  on  other  coasts  which  have  prevaihng  off- 
shore winds.  Bottom  currents  of  the  type  here  described  are 
probably  comparatively  feeble  as  a  rule;  but  under  favorable 
conditions,  as  in  channels  between  shallows  or  in  a  shallow  bay 
where  the  water  blown  in  can  only  escape  as  a  thin  bottom  layer, 
they  may  have  velocities  sufficient  to  move  fairly  coarse  debris, 


WIND  CUHRENTS  125 

especially  if  the  bottom  is  agitated  by  waves  which  keep  the 
debris  in  suspension.  Such  debris  would  then  migrate  land- 
ward with  an  offshore  wind,  and  seaward  when  the  wind  is  on- 
shore. 

Important  surface  currents  result  when  waters  heaped  up  by 
the  wind  against  a  coast  escape  to  either  side  along  the  shore,  or 
through  some  strait  into  an  adjacent  water  body.  Water  driven 
westward  across  the  Atlantic  by  the  trade  winds  piles  up  against 
the  western  shores  of  the  Caribbean  Sea,  raising  the  level  of 
the  Sea  above  that  of  the  Gulf  of  Mexico.  There  results  an 
hydrauKc  current  through  the  Strait  of  Yucatan  into  the  Gulf 
which  is  one  of  the  strongest  "of  known  ocean  currents,  having 
a  velocity  of  60  to  120  miles  a  day.  The  Gulf  of  Mexico,  in 
turn,  is  higher  than  the  Atlantic  Ocean,  and  hence  an  hydraulic 
current  passes  through  the  Florida  Strait  into  the  ocean  with  a 
velocity  of  70  to  100  miles  a  day.  This  is  the  beginning  of  the 
Gulf  Stream  proper,  which  off  Cape  Florida  is  only  15  miles  from 
the  shore  and  affects  the  bottom  to  a  depth  of  nearly  3000  feet. 
"  By  calculation  it  has  been  shown  that  a  current  of  the  velocity 
of  the  Gulf  Stream  requires  a  difference  of  elevation  of  at  least 
0.7  feet  of  the  Gulf  over  the  Atlantic,  which  difference  agrees 
very  nearly  with  that  found  by  direct  leveling  across  the  Florida 
Peninsula  "^^  A  line  of  levels  run  between  Cedar  Keys  on  the 
Gulf  coast  and  St.  Augustine  on  the  Atlantic  indicates  that  the 
difference  in  level  is  probably  at  least  0.8  feet^-.  A  current  of 
11  miles  per  day  flowing  eastward  through  the  strait  l^etween 
Cape  Horn  and  the  South  Shetland  Islands  illustrates  how  direct 
wind  impact  and  hydraulic  forces  may  act  in  the  same  direction 
to  give  a  compound  current;  for  the  winds  of  this  region,  which 
drive  the  water  eastward  through  the  strait,  also  pile  water  up 
against  the  coasts  of  Chile  and  the  South  Shetlands,  whence  the 
escape  for  the  hydraulic  current  is  .also  eastward  through  the 
strait^^. 

Temporary  Currents.  —  In  addition  to  the  more  or  less  per- 
manent wind  currents  referred  to  above,  there  are  temporary 
currents  of  considerable  local  importance  which  result  whenever 
strong  winds  blow  several  days  in  a  given  direction.  In  the  shal- 
low zone  along  a  coast  the  entire  mass  of  water  may  have  so 
strong  a  "  set  "in  the  direction  determined  by  the  wind  that  one  can 
readily  note  longshore  transportation  of  bottom  debris.    If  the  wind 


126  CURRENT  ACTION 

blows  directly  on  shore,  the  temporary  head  may  be  sufficiently 
great  to  produce  hydraulic  currents  of  no  mean  importance.     Thus 
during  a  furious  gale  on  the  15th  of  January,  1818,  the  water 
in  the  Kattegat  "  rose  5f  feet  above  the  common  waterstand  "^*. 
Northwest  gales  raise  the  level  of  the  North  Sea  on  the  coast  of 
Holland  4  or  5  feet  above  the  normal  tide  heights,  while  an 
easterly  wind  raises  the  waters  of  the  Black  Sea  against  the  coast 
of  Bulgaria  two  feet  above  the  ordinary  level.     MitchelP^  esti- 
mated that  a  northeast  storm   blowing  the  shallow  water  of 
Long  Island  Sound  westward  toward  Hell  Gate  in  New  York 
Harbor  caused  a  rise  of  6  feet  at  the  latter  point,  and  of  4  feet 
on  the  open  coast.     During  the  severe  storm  oi  November  21, 
1900,  when  the  wind  attained  a  velocity  of  80  miles  per  hour  at 
Buffalo,  the  lake  level  was  raised  8.4  feet^^     On  the  Zuyder  Zee, 
with  heavy  west  winds  the  water  is  lowered  8  feet  on  the  west 
coast,  and  raised  correspondingly  with  east  winds.     Owing  to  a 
heavy  gale  from  the  northeast  in  December,  1904,  the  water  in 
the  southern  part  of  the  Baltic  Sea  was  raised  from  8  to  12  feet 
above  its  normal  leveP^     In  the  Galveston  storm  of  September 
8,  1900,  the  Gulf  waters  rose  20  feet  and  were  the  principal 
agent  of  destruction  in  the  city.     During  the  storm  of  October 
5,  1864,  on  the  coast  of  India,  the  water  was  raised  24  feet  at 
Calcutta^^.     Such  inequalities  of  levels  must  give  rise  to  hydraulic 
currents  of  greater  or  less  importance  depending  upon  the  configura- 
tion of  the  shoreline.     According  to  Harrington^^  hydraulic  cur- 
rents on  the  Great  Lakes,  resulting  from  the  disturbance  of  water 
levels  by  storm  winds,  attain  a  velocity  as  high  as  240  miles  a  day. 
Seasonal    Currents.  —  Intermediate    between    the    permanent 
currents  resulting  from  such  winds  as  the  trades,  and  the  tem- 
porary currents  due  to  local   storm  winds,  are  currents  which 
prevail  during  one  season  or  another  because  of  seasonal  variations 
in  the  winds.    Seasonal  variations  of  current  direction  must  in  turn 
result  in  seasonal  variations  in  the  direction  of  debris  transporta- 
tion, and  hence  in  the  size,  form,  and  position  of  beaches.     Thus 
the  beach  on  the  southwest  point  of  Baker  Island,  in  the  PacifiG 
Ocean  near  the  equator,  migrates  from  one  side  of  the  point  to  the 
other  with  the  seasonal  change  in  the  winds'"".     Seasonal  currents 
consist  of  true  wind  currents  and  of  hydraulic  currents  resulting 
from  the  piling  up  of  the  wind-driven  water  against  the  continents. 
We  may  form  some  idea  of  the  probable  importance  of  these 


WIND  CURRENTS  127 

currents  if  we  know  the  seasonal  inequalities  of  water  level  along 
different  coasts.  Harris  has  summarized  a  number  of  cases, 
showing  seasonal  inequalities  which  measure  from  a  few  inches 
to  several  feet^^^  It  appears  from  this  summary  that  the  north- 
erly winds  of  winter  blowing  across  the  Gulf  of  Mexico  produce 
low  water  at  Galveston  in  February,  while  the  southerly  summer 
winds  produce  high  water  in  October,  the  difference  in  height 
due  to  this  cause  being  1.5  feet.  In  winter  the  northeast  mon- 
soons blow  the  water  away  from  the  coasts  of  the  northern  In- 
dian Ocean,  and  the  southwest  monsoons  of  summer  raise  the 
level,  the  difference  varying  from  1.8  to  3.2  feet.  The  south- 
westerly winds  which  prevail  at  Panama  during  much  of  the 
year  raise  the  water  against  the  northern  shores  of  the  Gulf  of 
Panama  2  feet  higher  than  the  level  which  exists  when  the  north- 
easterly winds  of  February  blow  the  water  away  from  those 
shores.  In  general,  it  is  noted  that  sealevel  is  highest  at  most 
tidal  stations  in  summer  or  autumn,  and  lowest  in  winter  or 
spring;  and  since  winds  tend  to  blow  from  the  ocean  toward  the 
land  in  summer,  and  from  the  lands  out  to  sea  in  winter,  it  ap- 
pears that  the  piling  up  of  the  waters  against  the  lands  in  summer 
must  be  the  principal  cause  of  high  water  at  that  time,  rather 
than  an  expansion  of  the  oceanic  waters  due  to  high  summer 
temperatures,  which  at  most  could  scarcely  raise  the  ocean  level 
0.1ofafooti«2. 

It  is  evident  that  such  large  seasonal  inequalities  of  level  as 
have  been  noted  above  must  be  accompanied  by  currents  which 
vary  in  direction  or  intensity,  or  both,  with  changes  of  the  seasons. 
In  the  Strait  of  Bab-el-Mandeb  at  the  mouth  of  the  Red  Sea 
there  is  a  southeasterly  current  during  the  summer,  because  the 
summer  monsoons  of  the  northern  Indian  Ocean  blow  the  sur- 
face water  out  of  the  Gulf  of  Aden,  making  its  level  lower  than 
that  of  the  Red  Sea.  In  winter  the  current  in  the  strait  flows 
northwest,  because  the  winter  monsoons  raise  the  Gulf  level, 
and  because  the  Gulf  waters  are  less  saline  and  therefore  less 
dense  than  those  in  the  Red  Sea.  The  currents  in  the  Strait 
often  have  a  velocity  of  30  to  40  miles  a  day,  and  as  high  a 
figure  as  2|  knots  per  hour,  or  66  sea-miles  per  day,  has  been 
recorded^°^.  These  seasonal  variations  of  the  surface  currents 
interfere  with  the  circulation  due  to  differences  of  salinity  which 
^would  otherwise  cause  a  constantly  inflowing  current  on  the 


128  CURRENT  ACTION 

surface  and  as  constant  an  outflowing  bottom  current.  As 
v/ith  other  currents  produced  by  the  wind,  the  direction  and 
intensity  of  the  seasonal  wind  currents,  and  of  their  resulting 
hydrauHc  currents,  depend  on  a  number  of  factors,  among  which 
wind  velocity,  water  depth  and  shore  configuration  are  of  highest 
importance.  There  is  a  summer  current  northward  through 
Bering  Strait  because  southerly  summer  winds  raise  the  waters 
of  the  shallow  northern  part  of  Bering  Sea  above  the  level  of 
the  Arctic  Ocean,  and  the  form  of  the  shores  offers  a  northward 
escape  through  a  narrow  channel'"^.  In  this  case  the  currents  is 
probably  in  part  a  true  wind  current,  although  largely  hydraulic. 
It  will  generally  be  found  that  near  a  coast  the  direct  wind  cur- 
rents do  not  move  in  the  precise  direction  of  the  wind,  but  are 
deflected  by  the  trend  of  the  shore  which  introduces  more  or  less 
of  the  hydraulic  element  into  the  currents. 

Planetary  Currents.  —  There  exist  in  the  principal  ocean  basins 
gigantic  whirls  or  eddies  which  are  commonly  referred  to  col- 
lectively simply  as  "  the  ocean  currents."  The  principal  cause 
of  these  great  movements  of  the  oceanic  waters  is  now  known 
to  be  the  planetary  wind  systems  which  blow  over  their  surface, 
although  earlier  students  assigned  a  more  important  place  to 
differences  in  oceanic  temperatures.  It  should  be  noted,  how- 
ever, that  while  the  planetary  winds  constitute  the  prime  cause, 
and  we  may  therefore  appropriately  call  the  currents  "  planetary 
currents,"  many  other  factors  must  be  recognized  in  any  full 
explanation  of  their  origin.  Taking  the  North  Atlantic  cir- 
culation as  an  example,  we  find  the  southern  side  of  the  great 
whirl  driven  westward  by  the  trade  winds,  and  the  northern 
side  driven  eastward  by  the  prevailing  westerlies.  But  account 
must  also  be  taken  of  the  deflective  effect  of  obstructing  land 
masses;  of  the  constantly  operating  force  arising  from  the 
earth's  rotation  which  tends  to  deflect  currents  toward  the  right 
in  the  northern  hemisphere;  of  the  hydraulic  action  resulting 
when  the  waters  driven  westward  are  piled  up  against  the  Carib- 
bean and  Gulf  coasts;  of  the  large  amount  of  rain  and  river 
water  added  to  the  ocean  in  the  Gulf  region;  and,  if  we  follow 
Ekman^"^  and  Carpenter^^^  of  the  water  drawn  into  the  Gulf  by 
reaction  currents  (see  below),  and  of  the  sinking  cold  water  near 
the  poles  and  rising  warm  water  in  low  latitudes.  It  is  simpler 
to  treat  currents  of  such  complex  origin  in  connection  with  the 


PLANETARY   CURRENTS  129 

individual  elements  which  combine  to  form  them;  but  the 
planetary  currents  are  sufficiently  distinct  and  well  known  to 
require  brief  mention  as  such. 

Planetary  currents  may  have  a  fairly  high  velocity  under 
favorable  conditions,  as  has  already  been  noted  for  that  part 
of  the  North  Atlantic  circulation  called  the  Gulf  Stream.  In 
Florida  Strait  this  current  may  reach  a  velocity  of  4  miles  an 
hour,  which  is  sufficient  to  move  large  stones;  and  the  current 
in  the  Strait  of  Yucatan  has  an  even  greater  velocity.  Such 
velocities  are  exceptional,  however,  and  the  bottom  waters  of 
even  these  currents  move  more  slowly.  Furthermore,  planetaiy 
currents  are  usually  located  in  deeper  water  far  from  the  coast, 
and  can  therefore  have  little  effect  upon  the  shoreline.  The 
swift  current  of  the  Gulf  Stream  in  Florida  Strait  is  some  miles 
off  shore  and  is  separated  from  the  land  by  another  and  slower 
current  moving  in  the  opposite  direction.  As  KriimmeP"^  has 
pointed  out,  even  where  currents  of  this  type  do  come  in  direct 
contact  with  the  land  they  are  almost  always  completely  over- 
powered by  tidal  or  other  currents  of  much  greater  importance 
in  shoreline  processes. 

A  good  account  of  the  former  exaggerated  ideas  regarding 
the  geological  work  of  ocean  currents  will  be  found  in  Riihl's 
review  of  the  literature  relating  to  the  "  Morphologischen 
Wirksamkeit  der  Meerestromungen  "^°^.  While  Riihl  does  not 
discriminate  sufficiently  between  the  different  types  of  currents 
found  in  the  ocean,  it  is  evident  that  many  of  the  reports  to 
which  he  refers  deal  with  planetary  currents.  Pechuel-Loesche^"^ 
gives  an  interesting  discussion  of  the  conditions  which  render 
planetary  currents  unimportant  agents  on  shoreline  develop- 
ment, but  tends  to  underestimate  the  transporting  power  of 
currents,  and  apparently  does  not  distinguish  sufficiently  between 
the  less  important  planetary  currents  and  the  movements  due 
to  tides  and  other  forces  which  often  have  a  very  high  degree  of 
importance.  Those  tempted  to  ascribe  shore  forms  to  currents 
represented  on  charts  or  described  in  coast  survey  publications 
will  do  well  to  remember  that  currents  so  reported  are  usually 
studied  several  miles  from  shore  where  the  water  is  deep  enough 
to  be  important  for  navigation;  whereas  the  shallow  waters 
near  the  shore,  of  the  highest  importance  to  students  of  shore 
forms,  are   usually  very  imperfectly  examined,  if   at  all.     The 


130  CURRENT  ACTION 

fact  that  a  certain  current  is  observed  several  miles  off  a  coast 
is  no  indication  whatever  that  the  waters  near  the  shore  move  in 
the  same  direction.  In  shallow  water,  it  is  true,  a  planetary  cur- 
rent may  reach  and  scour  the  bottom;  and  it  has  been  stated 
that  such  action  is  talcing  place  under  the  Gulf  Stream  between 
Florida  and  Cuba,  and  on  Blake  Plateau  southeast  of  Georgia"". 
Indirectly,  these  currents  aid  wave  erosion  by  helping  to  distrib- 
ute the  finer  waste  of  the  lands  far  over  the  ocean  floor  in  water 
so  deep  that  it  cannot  readily  be  returned  to  the  shore  zone"^ 

Pressure  Currents.  —  The  weight  of  the  atmosphere  on  the 
surface  of  the  ocean  is  about  15  lbs.  to  the  square  inch,  or  about 
8f  tons  per  square  yard.  It  is  evident  that  if  atmospheric 
movements  remove  part  of  this  weight  in  one  place  and  increase 
it  in  another,  the  sea  surface  must  rise  where  the  pressure  is  par- 
tially relieved  and  sink  where  it  is  increased.  Lubbock  has 
shown  that  as  a  rule  a  rise  of  1  inch  in  the  barometer  causes  a 
depression  in  the  height  of  high  water  amounting  to  7  inches  at 
London,  and  11  inches  at  Liverpool"^,  while  Bunt  has  found  that 
a  similar  barometric  rise  produces  a  depression  of  13.3  inches 
in  the  tides  at  Bristol"^.  Since  these  differences  of  level  are 
usually  distributed  over  broad  areas,  under  the  continuous  ap- 
plication of  pressures  which  alter  but  gradually,  they  probably 
do  not  often  cause  currents  strong  enough  to  be  perceived.  But 
if  two  water  bodies  connected  by  a  strait  are  subjected  to  unequal 
pressure,  currents  may  be  produced  in  the  strait  which  have  a 
fairly  liigh  velocity.  Ekman  has  shown  that  if  the  barometer 
fall  30  millimeters  over  the  Baltic,  the  result  would  be  the  same 
as  if  the  water  in  the  Kattegat  had  risen  4  centimeters.  This 
would  be  sufficient  under  certain  conditions  to  reverse  the  sur- 
face stream  normally  flowing  out  through  the  connecting  strait, 
and  to  give  a  distinct  current  into  the  Baltic"^.  In  the  Gulf  of 
St.  Lawn-ence  a  difference  in  atmospheric  pressure  is  said  to 
produce  a  flow  of  water  from  the  area  of  higher  to  that  of  lower 
pressure,  and  to  produce  currents  through  the  inlets  connecting 
the  Gulf  with  the  ocean.  High  pressure  over  the  Gulf  of  Mexico 
when  there  is  low  pressure  over  the  ocean  outside  appreciably 
increases  the  velocity  of  the  Gulf  Stream"^  It  is  diflicult  in 
these  cases  to  make  sure  that  the  effects  noted  are  wholly  due 
to  differences  in  pressure  and  are  not  affected  to  some  extent 
by  winds  blowing  from  areas  of  high  toward  areas  of  low  pressure. 


SALINITY  CURRENTS  131 

Marked  differences  of  pressure,  so  distributed  over  water  bodies 
of  the  proper  form  and  arrangement  as  to  favor  the  production 
of  pressure  currents,  do  not  appear  to  be  sufficiently  frequent 
or  sufficiently  lasting  to  make  these  currents  of  more  than  local 
and  temporary  importance. 

Convection  Currents.  —  The  warming  of  sea  water  causes 
it  to  expand  and  become  fighter,  while  cooling  causes  greater 
density  and  hence  increased  weight.  Therefore,  if  one  portion 
of  the  ocean  is  warmed  or  cooled  more  than  another,  convection 
currents  might  be  produced  which  would  endeavor  to  restore 
a  condition  of  perfect  equilibrium.  The  planetary  currents,  as 
already  noted,  have  been  seriously  regarded  by  some  as  mainly 
the  result  of  unequal  heating  of  the  ocean.  There  is  fittle 
doubt  that  a  slow  exchange  of  polar  and  equatorial  waters  is 
favored  by  temperature  differences,  the  cold  polar  waters  sink- 
ing and  creeping  equatorward  in  depth,  while  the  warmed  equa- 
torial waters  flow  poleward  over  the  surface.  That  portion 
of  this  motion  due  to  temperature  conditions  is,  however, 
extremely  slow.  Marked  differences  of  temperature  at  sealevel 
exist  only  between  regions  widely  separated;  and  the  resulting 
differences  in  ocean  level  are  very  small,  since  the  greatest 
difference  of  specific  gravity  that  can  arise  in  the  ocean  from 
differences  of  temperature  is  about  as  1 :1.0043"*'.  Hence  the  con- 
vection currents  which  arise  must  be  very  feeble.  It  should  be 
noted,  furthermore,  that  the  heat  which  tends  to  make  sea  water 
lighter  by  expanding  it,  also  causes  evaporation  and  thereby  tends 
to  increase  the  water's  density.  The  effects  of  increased  tempera- 
ture may  often  be  more  than  counterbalanced  by  the  effects  of 
evaporation.  It  is  doubtful  whether,  even  in  the  case  of  a 
strait  connecting  two  bodies  of  water,  the  currents  arising  from 
temperature  differences  alone  are  ever  sufficiently  strong  to  be 
of  importance  in  shoreline  processes. 

Salinity  Currents.  —  The  specific  gravities  of  fresh  water  and 
sea  water  are  very  different,  the  relation  at  15°  C.  being  as 
1:1.027,  and  at  0°  C.  as  1:1.0283,  if  the  sea  water  contains  3i  per 
cent  of  salt'".  It  follows  that  anything  which  locally  dilutes 
the  sea  water,  or  which  locally  increases  its  salinity,  will  produce 
currents  which  may  have  a  very  high  velocity.  The  most 
important  causes  of  dilution  or  increase  in  salinity  of  the  sea  are 
rainfall,    the   outflow   of   river   water,    and   evaporation.     It   is 


132  CURRENT  ACTION 

evident  that  these  processes  must  produce  direct  changes  of 
level  in  addition  to  changes  of  specific  gravity.  Rainfall  and 
the  outflow  of  rivers  raise  the  sealevel,  while  evg,poration  lowers 
it.  Such  differences  of  level  must  result  in  currents  which  will 
combine  with  the  currents  due  to  differences  of  specific  gravity 
to  form  a  single  system  of  circulation,  in  which  the  higher,  lighter 
water  flows  toward  the  lower  and  denser  water  on  the  surface, 
at  the  same  time  that  the  denser  waters  move  along  the  bottom 
toward  the  region  of  water  less  dense.  We  will  call  the  currents 
of  this  system,  salinity  currents. 

Rainfall  is  not  equally  distributed  over  the  surface  of  the  ocean. 
The  equatorial  rain  belt  has  an  excess  of  precipitation,  and  the 
same  is  true  of  higher  latitudes  where  rains,  snows,  and  melting 
ice  contribute  a  large  amount  of  fresh  water  to  the  sea.  The 
two  intermediate  zones,  from  near  the  equator  to  about  40° 
north  and  south  latitude,  are  characterized  by  deficient  rainfaU. 
There  must  be  a  tendency,  therefore,  for  surface  currents  to 
move  from  both  low  and  high  latitudes  toward  the  intervening 
areas  of  small  precipitation.  Such  a  movement  in  the  open 
ocean  would  be  comparatively  slow,  and  must  be  largely  masked 
by  other  currents  of  greater  importance. 

Strongly  marked  differences  in  density  are  produced  when  ice 
melts  in  the  sea,  and  the  resulting  currents  should  be  well  de- 
veloped in  such  regions  as  around  the  ice  barrier  of  the  Antarctic 
continent.  Pettersson"^  and  Sandstrom^^^  have  miade  special 
studies  of  such  currents,  and  have  shown  that  the  melting  ice 
dilutes  the  surface  water  and  causes  an  outward  or  seaward  sur- 
face movement.  The  water  below  the  ice  is  cooled,  its  density 
thereby  increased,  and  it  sinks  to  the  bottom  and  flows  outward 
as  a  bottom  current.  Between  these  two  there  must  result  an 
inward  moving  zone  of  water  which  has  been  neither  cooled  nor 
diluted.  Pettersson  goes  so  far  as  to  attribute  an  important 
part  of  the  main  oceanic  circulation  to  ice-melting,  an  extreme 
view  not  shared  by  most  oceanographers.  Barnes^^''  has  in- 
vestigated the  value  of  ice-melting  currents  in  enabling  navi- 
gators to  locate  icebergs  from  a  distance;  but  it  has  not  yet 
appeared  that  currents  of  this  origin  are  of  importance  in  shore 
processes. 

Fresh  water  poured  into  the  ocean  by  a  large  river  raises  the 
sealevel  at  that  point  and  lowers  the  density  of  the  ocean  water. 


SALINITY   CURRENTS  133 

Surface  currents  tend  to  move  out  in  all  directions,  and  the 
bottom,  denser  water  to  creep  in  toward  the  river  mouth.  On 
an  open  coast  tho  surface  currents  may  be  strong  in  the  immediate 
vicinity  of  the  river's  mouth  but  at  greater  distances  the  move- 
ments must  be  relatively  feeble.  Where  rivers  empty  into  a 
gulf  or  bay  the  level  may  be  so  much  raised  as  to  cause  a  very 
strong  current  at  the  outlet  to  the  ocean.  The  unusual  strength 
of  the  Gulf  Stream  may  in  part  be  due  to  the  large  amount  of 
water  brought  into  the  Gulf  of  Mexico  by  the  Mississippi  and 
other  rivers.  Even  if  the  fresh  water  does  not  actually  raise 
the  level  of  the  Gulf,  it  must  prevent  a  lowering  of  that  level  by 
evaporation,  and  thus  cause  a  virtual  rise  relatively  to  the  ocean 
outside^-^  The  surface  currents  moving  from  the  Arctic  Ocean 
through  Denmark  and  Davis  Straits  into  the  Atlantic  are  prob- 
ably in  part  salinity  currents.  "  The  considerable  precipitation, 
the  influx  from  several  large  rivers,  and  especially  the  small 
evaporation,  all  go  to  maintaining  a  rather  low  density  for 
Arctic  waters  as  well  as  an  increased,  but  of  course  very  small, 
elevation  of  the  surface.  .  .  .  Doubtless  a  considerable  amount 
of  water  passes  as  an  undercurrent  from  the  Atlantic  into  the 
Arctic  through  the  straits  east  and  west  of  Iceland  "^^l  In  the 
Gulf  of  St.  Lawrence  the  waters  have  a  lower  density  and  higher 
surface  than  in  the  Atlantic,  and  a  surface  current  of  2  knots 
per  hour  through  Cabot  Strait  is  attributed  by  Harris,  in  part 
at  least,  to  this  factl''^  although  Dawson  thinks  the  influence  of 
the  St.  Lawrence  River  water  upon  currents  in  the  Gulf  is  apt 
to  be  exaggerated^-*. 

Salinity  Currents  at  the  Mouth  of  the  Baltic  Sea.  —  The  enor- 
mous influx  of  river  water  into  the  Baltic  Sea  causes  that  water 
body  to  be  almost  fresh  at  its  northern  end,  and  to  have  a 
low  density  throughout;  while  its  surface  is  generally  believed 
to  be  higher  than  the  mean  level  of  the  sea  outside.  On 
this  basis  we  should  expect  a  surface  current  passing  outward 
through  the  straits  at  the  mouth  of  the  Baltic,  and  an  under- 
current of  heavier,  salt  water  flowing  into  the  Baltic  along  the 
bottom.  Such  a  circulation  exists,  and  the  velocity  of  the 
outflowing  surface  stream  is  usually  given  as  1  to  2  knots  per 
hour  in  the  Kattegat,  but  may  be  double  this  along  the  Nor- 
wegian coast  of  the  Skagerack.  It  is  strongest  in  spring  and 
early  summer,  when  the  influx  of  fresh  water  into  the  Baltic 


134  CURRENT  ACTION 

is  at  its  maximum^-^.  As  Otto^^*'  has  pointed  out,  unless  prevented 
by  other  currents  of  greater  power,  such  a  circulation  would  re- 
sult in  the  shore  debris's  being  controlled  by  the  outward  flowing 
surface  current,  while  the  bottom  debris  in  deeper  water  would 
be  swept  in  the  opposite  direction  by  the  inflowing  bottom  cur- 
rent. Ekman^-^  attributes  the  deep  channel  in  the  Skagerack 
and  Kattegat  to  the  scouring  action  of  the  bottom  current,  which 
prevented  deposition  along  its  course  of  the  sediment  now 
covering  the  bottom  of  the  North  Sea;  but  he  thinks,  apparently 
without  sufficient  reason,  that  this  was  done  when  the  land 
was  higher  and  melting  ice  supplied  larger  volumes  of  outflowing 
waters.  Perhaps  a  more  probable  interpretation  is  that  the 
channel  represents  a  normal  river  valley,  submerged,  and  since 
kept  open  by  current  action. 

Pettersson^-^  has  questioned  the  existence  of  a  higher  surface 
level  in  the  Baltic  on  the  ground  that  accurate  measurements 
show  the  water  level  at  Ystad  and  Landsort  on  the  Baltic  coast 
to  be  .024  and  .023  meters  respectively  helow  the  mean  annual 
level  at  Varberg  on  the  shore  of  the  Kattegat;  while  Bjerknes 
and  Sandstromi29  contend  that  the  difference  in  density  between 
the  Baltic  water  and  that  outside  is  not  sufficient  to  account 
for  the  existing  currents  in  the  Belts  and  Kattegat.  It  should 
be  noted,  however,  that  the  currents  behave  in  a  manner  normal 
for  salinity  currents,  and  they  are  generally  interpreted  as  such. 
The  Black  Sea  receives  every  year  152  cubic  kilometers  (about 
36  cubic  miles)  more  fresh  water  than  escapes  by  evaporation. 
Strong  salinity  currents  therefore  exist  in  the  Bosphorus,  the 
outflowing  fresher  surface  stream  at  Constantinople  having  a 
velocity  of  123  centimeters  per  second,  or  over  2  knots  per  hour. 
At  a  depth  of  25  meters  the  heavier  salt  water  is  flowing  inward 
with  a  velocity  of  73  centimeters  per  second,  the  velocity  de- 
creasing slowly  with  increase  in  depth^'". 

Salinity  Currents  at  the  Strait  of  Gibraltar.  —  Evaporation  is 
an  effective  agent  in  producing  salinity  currents  but  in  this 
case  the  surface  current  must  of  course  flow  inward  toward 
the  region  of  evaporation,  where  the  water  is  increasing  in  den- 
sity and  the  surface  is  being  lowered;  while  the  heavier  salt 
water  will  flow  outward  at  a  lower  level.  A  striking  example 
of  such  circulation  is  found  in  the  Strait  of  Gibraltar.  The 
annual   evaporation   from    the   surface   of   the    Mediterranean 


SALINITY   CURRENTS  135 

amounts  to  a  layer  of  water  at  least  3  meters  deep  according  to 
Fischer^^^  and  greatly  exceeds  the  influx  of  fresh  water,  with  the 
result  that  the  waters  in  the  sea  become  denser  and  the  surface 
lower  than  is  the  case  in  the  Atlantic  Ocean.  The  higher  and 
lighter  waters  of  the  Atlantic  flow  into  the  Mediterranean  as  a 
surface  stream  of  marked  strength,  while  deep-water  obser- 
vations prove  that  a  strong  current  of  more  saline  water  moves 
outward  on  the  bottom.  The  great  velocity  of  these  currents  is 
a  matter  of  considerable  interest.  Maury^^-  quotes  the  following 
from  the  abstract  log  of  Lieutenant  W.  G.  Temple  for  March 
8,  1855,  relating  to  the  inflowing  surface  current:  "Weather 
fine;  made  Ij  pt.  leeway.  At  noon,  stood  in  to  Almiria  Bay,  and 
anchored  off  the  village  of  Roguetas.  Found  a  great  number 
of  vessels  waiting  for  a  chance  to  get  to  the  westward,  and  learned 
from  them  that  at  least  a  thousand  sail  are  weatherbound  be- 
tween this  and  Gibraltar.  Some  of  them  have  been  so  for  six 
weeks,  and  have  even  got  as  far  as  Malaga,  only  to  be  swept 
back  by  the  current.  Indeed,  no  vessel  had  been  able  to  get 
out  into  the  Atlantic  for  three  months  past."  It  would  seem 
from  this  that  the  surface  salinity  current,  reinforced  no  doubt 
by  an  hydraulic  current  due  to  heaping  up  of  water  in  the  Gulf 
of  Cadiz  under  westerly  winds^^^,  and  perhaps  also  to  some  ex- 
tent by  a  direct  wind  current,  had  a  velocity  sufficiently  great 
to  prevent  sailing  vessels  from  passing  westward  to  the  Atlantic 
for  months  at  a  time.  Helland-Hansen^^^  has  shown  that  tidal 
currents  also  affect  the  movement  of  the  waters  in  the  strait, 
the  direction  of  flow  at  a  depth  of  10  meters  even  being  reversed 
from  its  usual  inward  course  for  a  brief  period  on  the  day  of  his 
observations.  The  maximum  velocity  of  the  inflowing  current 
at  a  depth  of  10  meters  was  on  that  day,  118  centimeters  per 
second,  or  2.3  knots  per  hour.  On  another  day  the  velocity  of 
the  inflowing  current  at  a  depth  of  5  meters  was  150  centimeters 
per  second,  or  nearly  3  knots  per  hour.  At  a  depth  of  46  meters 
the  inflowing  current  had  a  velocity  of  1.8  knots,  and  at  a  depth 
of  91  meters  a  velocity  of  2  knots.  The  depth  for  the  next  series 
of  observations  was  183  meters  (100  fathoms)  and  both  here  and 
below  the  current  was  continuously  flowing  out  into  the  Atlantic. 
On  the  surface  the  current  nearly  always  flows  inward  with  a 
velocity  of  about  3  knots  per  hour^^^. 

The  strength  of  the  outflowing  bottom  current  is  more  re- 


136  CURRENT  ACTION 

markable  than  that  of  the  inflowing  surface  current.  With 
274  meters  (150  fathoms)  of  wire  out  the  exact  depth  could  not 
be  learned  because  the  wire  was  so  strongly  bowed  by  the  force 
of  the  current.  A  maxinunn  velocity  of  227  centimeters  per 
second,  or  4.4  knots  per  hour,  was  recorded.  When  sent  down 
with  366  meters  of  wire  the  apparatus  was  wrecked,  apparently 
by  being  bumped  against  stones  on  the  l^ottom'^''.  Sir  James 
Anderson  has  stated  that  the  velocity  of  this  outflow  is  so  great 
at  the  bottom  that  at  a  depth  of  500  fathoms  the  wire  of  the 
Falmouth  cable  near  Gibraltar  was  ground  like  the  edge  of  a 
razor,  so  that  it  had  to  be  abandoned  and  a  new  one  laid  well 
inshore^^^  Captain  Nares  reports  that  he  could  get  no  specimen 
of  the  bottom,  probal)ly  because  of  a  "  perfect  swirl  at  that 
depth  "i'^^  Such  currents  must  be  very  effective,  not  only  in 
scouring  the  bottom  at  great  depths,  but  also  in  transporting 
to  a  final  resting  place  in  very  deep  water  any  debris  which  may 
be  delivered  to  it  by  the  agitated  surface  waters. 

Salinity  Currents  at  the  Strait  of  Bab-el- Mandeb.  — Important 
salinity  currents  due  to  evaporation  occur  in  the  Strait  of 
Bab-el-Mandeb  at  the  mouth  of  the  Red  Sea.  This  sea  is 
located  in  one  of  the  dryest  regions  of  the  world,  and  possesses 
the  highest  mean  annual  salinity  of  any  body  of  water  in  com- 
munication with  the  open  ocean^^l  As  a  consequence  there 
is  an  inflow  of  lighter  water  on  the  surface  of  the  strait  and  an 
outflow  of  heavy  salt  water  on  the  loottom,  except  when  this 
circulation  is  interfered  with  by  hydraulic  currents  caused  l^y 
the  monsoon  winds.  The  velocity  of  the  inflowing  current 
is  variously  stated  as  from  30  to  65  knots  per  day,  or  a  maxi- 
mum of  about  2|  knots  per  hour"".  The  outflowing  bot- 
tom current  varies  from  1  to  3  knots  per  hour.  Even  the  lowest 
velocity  mentioned  for  either  stream  is  sufficient  to  move  fine 
gravel;  and  it  cannot  be  doubted  that  currents  of  this  type 
play  an  important  role  along  the  shores  of  straits  and  the  narrow 
parts  of  adjacent  seas,  even  though  the  swiftest  current  is  never 
found  in  the  immediate  vicinity  of  the  shoreline. 

River  Currents.  —  Rivers  entering  the  sea  have  their  currents 
checked  before  they  have  advanced  far  into  the  quieter  water, 
and  in  place  of  a  narrow  stream  of  fresh  water  moving  forward 
under  the  impetus  of  the  river's  original  velocity,  there  are  de- 
veloped slower  hydraulic  currents  due  to  the  pihng  up  of  the 


RIVER  CURRENTS  137 

waters,  salinity  currents  due  to  differences  in  specific  gravity, 
and  reaction  and  eddy  currents  generated  by  the  dynamic  force 
of  the  original  stream.  For  a  short  distance,  however,  one  may 
recognize  the  true  river  current,  the  extent  of  its  penetration  as 
such  into  the  sea  depending  on  the  volume  and  velocity  of  the 
river,  the  form  of  the  shore  and  sea-bottom,  and  other  factois. 
Dall"^  has  attributed  the  clockwise  circulation  of  the  Bering  Sea 
in  part  to  river  currents  which  enter  the  eastern  side  of  the  sea 
with  a  southwestward  trend. 

As  the  river  current  is  checked  by  contact  with  the  quieter 
waters  of  the  sea  it  must  of  course  deposit  the  debris  it  is  trans- 
porting, the  coarsest  first,  the  finer  as  the  current  grows  more  and 
more  sluggish.  If  the  river  is  heavily  laden  with  sediment,  and 
the  water  body  into  which  it  empties  is  not  greatly  agitated 
by  other  types  of  currents,  much  of  the  debris  will  remain  where 
first  dropped  to  form  a  delta.  Most  rapid  deposition  occurs 
beneath  and  along  the  immediate  margins  of  the  river  current, 
with  the  result  that  the  current  is  ultimately  confined  between 
walls  of  its  own  deposits  and  prevented  from  coming  in  contact 
with  the  adjacent  waters  until  it  has  passed  beyond  the  limits 
of  the  embankments.  Thus,  the  river  current  is  carried  farther 
and  farther  out  into  the  oceanic  waters  between  the  two  sides  of 
an  elongating  delta  lobe,  as  in  the  case  of  the  Mississippi  delta, 
some  lobes  of  which  have  advanced  into  the  Gulf  of  Mexico  at 
the  rate  of  from  one  hundred  to  several  hundred  feet  a  year. 
On  the  other  hand,  if  a  strong  current  of  any  type  sweeps  along 
the  coast  opposite  the  mouth  of  a  river,  the  river  current  may  be 
deflected  so  as  to  merge  with  the  longshore  current.  As  the  river 
current  gradually  loses  its  identity  the  sediment  is  carried  on 
by  the  higher  velocity  of  the  more  powerful  longshore  current. 
Under  these  conditions  no  delta  will  form.  Assuming  that  a 
river  brings  down  a  significant  amount  of  sediment,  its  ability 
to  form  a  delta  does  not  depend  upon  its  entering  a  tideless  sea, 
as  is  usually  stated,  but  rather  upon  its  entering  a  comparatively 
currentless  sea.  The  Indus  builds  its  delta  in  a  sea  having  a 
tidal  range  of  10  feet,  while  the  Ganges  delta  has  formed  where 
the  range  is  16  feet.  The  theory  that  deltas  are  restricted  to 
tideless  seas  is  fallacious.  If  the  river  current  is  stronger  than 
other  currents  in  the  sea  at  the  point  of  its  embouchure,  and  in 
consequence  is  carrying  debris  which  those  other  currents  can- 


138  CURRENT  ACTION 

not  transport,  a  delta  will  form,  whatever  may  be  the  tidal  range. 
If  the  strength  of  any  other  type  of  current  exceed  that  of  the 
river  current  at  the  point  of  embouchure,  no  delta  will  form. 

It  often  happens  that  the  river  current  is  strongest  at  the 
immediate  point  of  embouchure  to  begin  with,  although  a  more 
powerful  current  sweeps  along  the  coast  some  distance  out  in 
the  sea.  This  is  especially  apt  to  be  the  case  where  a  river  enters 
the  sea  at  the  head  of  a  reentrant  angle  or  bay.  Delta  formation 
will  then  proceed  until  the  river  current  has  been  carried  seaward, 
by  the  advancing  delta  lobe,  to  the  point  where  it  conflicts  with 
and  is  overcome  by  the  longshore  current.  Such  seems  to  have 
been  the  history  of  the  Nile  delta;  for  although  the  river  cur- 
rent brings  out  to  sea  36,600,000  cubic  meters  of  silt  annually, 
this  vast  tribute  of  sedime\it  does  not  add  to  the  seaward  extent 
of  the  delta,  because  "  a  powerful  marine  current  sweeps  past  the 
coast  and  carries  the  sediment  eastward  beyond  the  most  easterly 
mouth  of  the  river  ""'.  Much  of  the  sediment  now  brought 
down  by  the  Amazon  is  carried  seaward  with  the  aid  of  strong 
tidal  currents,  then  caught  up  by  the  Northern  branch  of  the 
South  Equatorial  current,  which  transports  part  of  it  over  300 
miles  to  deposit  it  along  the  coast  of  Guiana^^^  The  forms  of 
deltas  will  evidently  depend  not  only  upon  the  original  form  of 
the  shoreline,  the  nature  and  quantity  of  the  debris  brought 
down  by  the  river,  and  the  manner  in  which  the  river  shifts  its 
position  upon  the  delta;  but  also  upon  the  extent  of  wave  action, 
and  the  direction  and  strength  of  coastal  currents  of  different 
types  as  compared  with  the  strength  and  direction  of  the  river 
current . 

Reaction  Currents.  —  F.  L.  Ekman"^  has  shown  that  since  a 
river  flowing  into  the  sea  sets  adjacent  water  particles  moving 
forward  in  the  same  direction,  thereby  increasing  the  volume  of 
the  current  more  rapidly  than  its  velocity  is  decreased,  there 
must  be  an  influx  of  seawater  toward  the  mouth  of  the  river  to 
make  good  the  resulting  deficiency.  "  Every  river  or  brook 
which  falls  into  the  sea  gives  rise  to  an  undercurrent  directed 
toward  its  embouchure.  These  undercurrents  are  so  distinct 
and  the  causes  that  produce  them  so  active,  that  in  calm  weather 
their  presence  may  be  easily  observed  at  the  mouth  of  the  most 
insignificant  rivulet  that  falls  out  over  the  surface  of  the  sea." 
To  such  currents  Ekman  gives  the  name  "  reaction  streams." 


EDDY  CURRENTS  .  139 

Cornish  has  called  them  "  induction  currents  "^*K  Investigations 
of  the  outlet  of  the  Gota-Elf  into  the  Kattegat  showed  that  a 
reaction  current  flowed  well  into  the  bed  of  the  river  as  a  dis- 
tinct bottom  current  of  salt  water.  A  sunken  object  was  moved 
up  the  river  channel  by  this  current,  in  direct  opposition  to  the 
surface  flow"".  It  was  shown  that  this  current  could  not  be 
explained  as  a  mere  salinity  current  due  to  differences  of  specific 
gravity  between  the  fresh  and  salt  water.  Ekman  even  goes 
further,  and  regards  the  bottom  currents  at  the  outlet  of  the 
Baltic  Sea  and  in  the  Strait  of  Gibraltar  as  in  large  part  reaction 
currents.  Cronander^^^  on  the  other  hand,  would  seem  to  doubt 
the  existence  of  true  reaction  currents,  even  at  the  outlet  of 
the  Gota-Elf  where  Ekman  made  his  principal  study.  While 
there  are  probably  reaction  currents  developed  both  at  the 
mouth  of  the  Baltic  and  at  the  inlet  to  the  Mediterranean,  Ek- 
man seems  to  push  his  theory  too  far  and  to  lose  sight  of  the 
facts  that  salinity  currents  of  large  volume  must  exist  under  the 
conditions  obtaining  at  such  straits  as  those  in  question,  and  that 
any  reaction  currents  found  there  are  secondary  phenomena  of 
less  importance  than  the  currents  which  give  rise  to  them. 
Reaction  currents  have  been  further  studied  by  V.  W.  Ekman, 
the  son  of  the  investigator  quoted  above,  and  some  of  his  con- 
clusions are  embodied  in  a  valuable  paper"^  published  in  1899. 
According  to  his  studies,  reaction  currents  are  not  always  well 
developed  at  the  mouths  of  rivers,  and  may  even  fail  entirely"''. 
On  the  other  hand,  Buchanani^o  goes  so  far  as  to  explain  the  sub- 
marine gorge  opposite  the  mouth  of  the  Congo  as  due  to  reaction 
currents,  which  prevented  sedimentation  in  the  seaward  prolonga- 
tion of  the  river's  course  while  the  continental  shelf  on  either  side 
was  being  built  up. 

Theie  can  be  little  doubt  that  reaction  currents  must  have 
some  effect  upon  the  transportation  of  debris  in  the  vicinity  of 
river  mouths,  and  possibly  in  other  localities.  But  while  bot- 
tom debris  has  been  observed  in  motion  under  the  influence  of 
these  currents,  our  knowledge  of  their  geological  work  and  its 
relative  importance  is  very  slight. 

Eddy  Currents.  —  Closely  related  to  the  reaction  currents 
described  above  are  the  eddy  currents,  which  also  result  from 
the  dynamic  force  exerted  by  the  moving  waters  of  currents 
of  pther  types.     In  the  typical  reaction  current  the  water  moves 


140 


CURRENT  ACTION 


in  under  the  original  current  which  produced  it.  Eddy  cur- 
rents (called  '' di-aught  currents"  by  Bache),  on  the  other  hand, 
are  surface  whirls  in  which  the  water  next  the  original  current 
moves  forward  beside  it,  the  opposite  side  of  the  whirl  flowing 
in  the  reverse  direction.  Thus  the  clockwise  planetary  whirls 
of  the  northern  oceans  give  rise  to  counter-clockwise  eddies 
on  their  outer  sides.  The  surface  manifestations  of  these  whirls 
are  so  well  known  that  it  seems  desirable,  notwithstanding  their 
close  affinity  with  the  reaction  currents,  to  treat  them  separately 
under  the  name  of  eddv  currents. 


Fig.  21.  —  Eddy  currents  in  the  Gulf  of  Honduras  and  Mosquito  Gulf. 

The  salinity  current  entering  the  Mediterranean  Sea  moves 
eastward  along  the  northern  coast  of  Africa,  aided  by  the  prevail- 
ing westerly  winds.  In  the  gulf  off  the  coast  of  Tripoli  it  causes 
a  well  marked  eddy  current.  The  Equatorial  Current  flowing 
through  the  Caribbean  Sea  produces  one  eddy  current  in  the 
Gulf  of  Honduras  and  another  in  the  larger  embayment-  of 
Mosquito  Gulf  north  of  Panama  (Fig.  21).  Tidal  currents  enter- 
ing New  York  Harbor  cause  an  edd}'  current  just  inside  of  Sandy 
Hook  which  must  afTect  the  development  of  that  spit^^^  Gul- 
liver has  shown  that  eddy  currents  developed  by  tidal  currents 
in  estuaries  may  help  to  determine  the  detailed  form  of  the 
shoreUne^^^^  and  Abbe  has  even  attributed  the  formation  of  the 


COMPLEXITIES  OF  CURRENT  ACTION  141 

great  Carolina  capes  to  eddy  currents  generated  by  the  Gulf 
Stream^^^.  A  great  deal  of  importance  has  been  attributed 
the  Florida  counter-current  in  determining  the  shore  forms 
to  along  the  eastern  and  southern  coasts  of  that  peninsula^^. 
While  there  may  be  some  question  as  to  the  origin  of  this  cur- 
rent, and  some  even  doubt  whether  its  existence  has  been  fully 
estabhshed,  Perkins^^^  is  of  the  opinion  that  in  so  far  as  it  is  a 
reality  it  is  probably  an  eddy  current  generated  by  the  Gulf 
Stream. 

Deflection  of  Currents.  —  All  of  the  currents  above  described 
are  subject  to  the  deflective  effect  of  the  earth's  rotation.  Those 
in  the  northern  hemisphere  are  deflected  to  the  right,  those  in 
the  southern  hemisphere  to  the  left.  The  deflection  is  un- 
recognizable in  short,  temporary  currents,  such  as  those  arising 
from  wave  action;  but  is  prominently  shown  by  large  con- 
tinuously moving  currents,  like  those  of  the  planetary  circulation, 
and  may  even  be  observed  in  the  smaller  salinity  currents  and 
other  similarly  restricted  circulations. 

COMPLEXITIES  OF  CURRENT  ACTION 

The  preceding  discussion  of  the  several  types  of  currents 
encountered  in  the  sea  is  sufficient  to  show  that  the  subject  is  by 
no  means  a  simple  one.  We  have  endeavored  to  analyze  the 
origin  and  nature  of  each  type  separately,  and  to  gain  some  idea 
of  its  probable  relative  importance.  But  we  fully  realize  that 
in  nature  one  seldom  encounters  one  of  these  currents  operating 
alone.  In  almost  every  case  the  ocean  water  moves  in  a  given 
direction  because  of  the  combined  influence  of  several  forces. 
At  the  Strait  of  Gibraltar  the  inward  surface  flow  may  at  a  given 
moment  represent  the  combined  effect  of  salinity,  wind,  hydraulic, 
tidal,  pressure,  and  reaction  currents,  all  moving  in  the  same 
direction.  Off  Storeggen  movements  of  the  water  toward  the 
northeast  were  found  to  result  from  the  combined  action  of 
planetary  and  tidal  currents^^".  Along  the  south  coast  of  Alaska 
a  prominent  planetary  or  eddy  current  and  the  local  tidal  currents 
are  so  far  affected  by  wind  currents  that  it  has  been  asserted  that 
"  the  currents  along  this  part  of  the  coast  are  controlled  entirely 
by  the  winds  "^^^  The  currents  in  the  Strait  of  Bab-el-Mandeb 
are  variable  in  character  because  salinity,  wind,  and  hydraulic 


142  CURRENT  ACTION 

currents  combine  in  varying  proportions  at  different  times  of 
the  year;  and  they  are  further  "  confused  through  the  irregular 
tidal  influence  felt  there  "^^^.  Tidal  currents  on  the  south  coast 
of  Cantyre  are  uncertain  and  imperfectly  understood,  being 
much  affected  by  wind  currents^^^.  Ekman  has  described  the 
complex  nature  of  the  Gulf  Stream''^'*;  Buchan  and  Ekman  have 
both  discussed  at  length  the  combined  effects  of  salinity  and 
temperature  on  oceanic  circulation"^^;  and  Parsons  has  described 
the  combination  of  tidal  and  hydraulic  currents  in  New  York 
Harbor,  and  mentioned  the  difficulties  arising  from  the  interfering 
action  of  salinity,  wind,  and  eddy  currentsi'^l  Wind,  pressure, 
and  hydraulic  currents  may  combine  to  reverse  the  normal 
outflowing  salinity  current  at  the  mouth  of  the  Baltic^^^  while 
the  similar  current  out  of  the  Black  Sea  is  reversed  during  strong 
so-uth  winds^"^.  Cronander  even  goes  so  far  as  to  reject^the  com- 
monly accepted  theory  of  salinity  currents  at  the  mouth  of  the 
Baltic,  and  regards  both  surface  and  bottom  currents  as  due  to 
the  wind^*'^  The  continuous  outflowing  current  just  inside  the 
northern  end  of  Sandy  Hook  is  part  of  the  time  a  true  tidal 
ebb  current,  and  part  of  the  time  an  eddy  current  developed  by 
the  flood  tide'''^.  Otto  has  shown  the  difficulty  of  analyzing  the 
movement  of  shore  and  bottom  debris  by  currents  along  the 
south  shore  of  the  Baltic^^''. 

Further  complication  arises  from  the  fact  that  along  the  same 
shore  different  types  of  currents  may  act  with  very  different 
strengths,  and  the  same  current  may  have  very  different  power 
in  two  adjacent  areas.  In  the  shallow  water  close  to  the  beach, 
wind  and  wave  currents  are  extremely  effective,  while  tidal 
currents  may  be  scarcely  perceptible.  A  few  yards  out  from 
the  same  shore,  in  water  of  moderate  depth,  a  tidal  current  may 
sweep  with  irresistible  force,  while  the  wave  current  will  be  too 
feeble  on  the  bottom  to  move  coarse  debris.  Divers  have  found 
that  while  large  surface  waves  will  not  interfere  with  their  work 
on  the  bottom,  a  tidal  current  may  sweep  so  strongly  over  the 
same  spot  that  it  becomes  impossible  to  stand  against  it^^^ 
Let  us  imagine  that  in  such  a  case  the  wind  current  and  beach 
drift  is  toward  the  east,  while  the  tidal  current  runs  toward  the 
west.  At  the  shore  one  observer  notes  that  throughout  the 
year  the  wind  and  waves  invariably  cause  the  shingle  to  be 
moved  visibly  eastward.     Another  observer  finds  that  the  only 


COMPLEXITIES  OF   CURRENT  ACTION  143 

known  source  of  supply  for  the  rocks  from  which  the  shingle  is 
derived  lies  to  the  east,  and  hence  concludes  that  tidal  currents 
transport  the  material  westward.  Both  are  right,  for  the  tidal 
current  carries  the  shingle  westward  so  long  as  it  remains  in 
deep  water;  but  as  fast  as  part  of  the  material  is  moved  into 
shallow  water,  or  is  thrown  upon  the  beach  by  unusually  large 
storm  waves,  it  comes  under  the  influence  of  the  eastward  di- 
rected forces;  and  it  continues  to  move  in  this  direction  so  long 
as  it  is  not  washed  back  into  deeper  water  where  the  westward 
moving  tidal  current  prevails. 

Conflicting  Opinions  Regarding  Current  Action.  —  A  brief  ex- 
amination of  the  literature  is  sufficient  to  show  that  in  cases 
similar  to  the  one  supposed  above,  one  observer  has  frequently 
denied  the  validity  of  another's  interpretation  at  the  same  time 
that  he  maintained  the  correctness  of  his  own.  The  engineers 
and  other  authorities  in  Great  Britain  have  of  necessity  paid 
much  attention  to  the  problems  of  coast  erosion  and  transporta- 
tion; and  if  one  looks  through  some  of  the  papers  on  this  subject 
published  in  "  Minutes  of  Proceedings  of  the  Institution  of  Civil 
Engineers,"  he  will  be  surprised  at  the  wide  differences  of 
opinion  there  expressed  by  different  experts,  on  the  question  as 
to  what  agent  effects  the  longshore  transportation  of  sand  and 
shingle.  Discussions  on  this  point  cover  many  pages  and  some- 
times required  the  entire  time  of  two  or  more  meetings  for  their 
consideration.  According  to  the  views  expressed,  both  in  these 
discussions  and  before  other  learned  societies,  the  transportation 
of  shingle  is  due  "  chiefly,  if  not  entirely,  to  the  action  of  wind 
waves"  (J.  Scott  Russell);  "to  the  effects  of  the  ocean-wave  or 
ground-swell"  (J.  N.  Douglas),  since  "  waves  possessed  sufficient 
power  to  move  shingle  at  considerable  depths  "  (Joshua  Wilson), 
or  even  "at  very  great  depths"  (E.  Belcher);  whereas  "very 
little  was  ascribable  to  action  of  the  tide  "  (G.  B.  Airy),  for  "  the 
tide  current  does  not  affect  the  depths  of  more  than  12  or  14 
feet"  (E.  Belcher),  and  "the  tidal  streams  had  not  sufficient 
velocity  to  exercise  any  mechanical  power  whatever  "  (R.  A.  C. 
Austen).  On  the  other  hand,  we  have  the  opinions  that  "shingle 
could  scarcely  be  moved  by  the  heaviest  waves  at  greater  depth 
than  three  fathoms  "  (J.  M.  Rendel) ;  the  formation  of  the  great 
shingle  deposit  of  the  Dungeness  "  should  be  attributed,  princi- 
pally, to  the  counter-current  of  the  tides  "  (G.  Rennie);  and  "  at 


144  CURRENT  ACTION 

Cahore  the  driftage  is  solel}'  due  to  the  fiow-tide  currents " 
(G.  H.  Kinahan),  while  the  movement  of  another  shingle  beach 
was  due  to  "submarine  currents  which  had  the  power  of  carry- 
ing pebbles  along  the  shore" at  great  depths"  (Joseph  Gibbs). 
As  Hunt^'^^  has  pointed  out,  although  "the  action  of  waves  on 
sea-beaches  and  sea-bottoms  has  been  much  discussed  during  the 
last  fifty  years,  .  .  .  there  is  scarcely  an  important  point  con- 
nected with  the  subject  that  is  accepted  without  dispute,  whilst 
not  only  the  opinions,  but  even  the  recorded  observations  of 
skilled  observers  are  often,  to  all  appearance,  in  hopeless  conflict.'' 

Not  only  the  cause  of  shingle  transportation,  but  also  such 
questions  as  whether  large  or  small  debris  travels  farthest,  and 
under  what  conditions  waves  build  up  or  destroy  shingle  beaches, 
are  in  dispute.  According  to  Coode™  large  pebbles  travel  far- 
thest because  they  move  more  readily  than  small  ones;  Redman^^i 
agrees  to  the  greater  travelling  power  of  the  large  material,  as 
does  also  Reade^",  who  rejects  Coode's  explanation,  however, 
and  suggests  one  of  his  own.  On  the  other  hand  Prestwich^^^, 
Palmer"^,  Airy^^^,  Spratt^^^  and  Geikie^"  hold  that  the  smaller 
pebbles  are  those  which  travel  farthest. 

The  question  as  to  whether  the  shingle  travels  east  or  west  on 
the  great  Chesil  Bank  of  the  south  coast  of  England  has  long 
been  disputed,  with  eminent  authorities  on  both  sides.  Whether 
the  largest  or  smallest  pebbles  tend  to  accumulate  at  the  top  of 
the  beach  has  likemse  been  vigorously  debated.  Coode^^^,  Mat- 
thews"^, and  Shield^*''  state  that  with  offshore  winds  the  waves 
build  up  shingle  beaches,  while  with  onshore  winds  the  beaches 
are  cut  away;  but  Kinahan^^^  is  of  the  opinion  that  the  reverse  is 
the  case.  Palmer^^-  concluded  that  when  more  than  ten  breakers 
arrived  in  a  minute  the  beach  was  eroded,  when  less  than  ten, 
the  beach  was  built  up;  but  Coode^^^  declares  that  so  far  as  a 
rule  can  be  established  it  is  that  any  number  of  breakers  greater 
than  nine  per  minute  causes  the  building  up  of  the  beach,  while 
seven  or  less  produces  erosion. 

Reasons  for  Conflicting  Opinions.  —  The  remarkable  disagree- 
ment which  has  been  illustrated  above  is  not  so  surprising  when 
one  considers  the  complex  origin  of  currents  in  the  sea,  and  the 
enormous  variability  of  wave  and  tidal  action  along  a  coast. 
There  can  be  no  doubt  that  in  some  localities  tidal  currents  play 
a  more  important  role  in  the  longshore  transportation  of  sand 


COMPLEXITIES  OF  CURRENT  ACTION  145 

and  shingle  than  do  wave  currents,  beach  drifting,  and  related 
forces;  and  it  is  equally  certain  that  in  many  other  localities 
the  currents  associated  with  wave  action  are  more  important 
transporting  agents  than  are  those  of  tidal  origin.  In  still  other 
localities  it  may  be  difficult  to  determine  which  of  these  two 
types  of  currents  exercise  a  predominant  influence  upon  the 
shoreline,  or  whether  some  other  current  may  not  be  more 
important  than  either.  The  present  writer  entertains  no  doubt 
that  as  a  whole  waves  are  far  more  important  agents  of  long- 
shore movement  of  beach  material  than  are  tides  or  other  forces. 
It  does  not  appear  that  the  conclusions  of  the  authorities 
quoted  above  where  based  on  any  adequate  analysis  of  the 
complex  forces  operating  along  the  shore.  On  the  contrary,  in 
a  large  number  of  the  instances  cited  conclusions  were  based  on 
isolated  observations  in  a  limited  number  of  places,  and  while 
these  observations  were  usually  made  with  skill  and  accuracy, 
they  were  utterly  inadequate  as  a  basis  for  general  conclusions 
concerning  such  difficult  problems  as  those  encountered  at  the 
shoreline.  Erroneous  ideas  as  to  the  strength  of  certain  cur- 
rents have  crept  into  standard  textbooks,  as  for  example  Reade's 
conclusions  regarding  the  strength  of  tidal  currents  near  Gibraltar 
based  on  observations  which  really  related  to  salinity  currents^^. 
This  is  inevitable,  in  view  of  the  limited  knowledge  of  ocean 
currents  which  exists  even  to  the  present  time.  Again,  the 
resemblance  between  certain  currents  of  different  origin  is  so 
close  that  special  care  must  be  taken  properly  to  distinguish 
them.  Thus,  at  the  mouth  of  a  river  we  may  have  a  landward 
directed  bottom  current  which  may  be  a  salinity  current,  a 
reaction  current,  or  a  floodtide  current,  or  all  of  these  combined. 
Mitchell ^^  describes  such  a  landward  current  at  the  mouth  of  the 
Hudson  River,  and  regards  it  as  a  true  flood-tide  current  which 
cr(;eps  in  along  the  bottom  because  it  is  heavier  than  the  brack- 
ish water  in  the  river.  Harris^*^  refers  to  this  same  current  as  one 
of  the  "  counter  currents  at  the  bottom  of  the  channel  "  caused 
by  "  a  fresh-water  stream  discharging  into  the  ocean,"  and  refers 
to  Mitchell's  work  apparently  under  the  impression  that  IMit- 
chell  regarded  the  movement  as  a  reaction  current.  It  seems  to 
the  present  writer  that  the  conditions  in  this  water  body  are 
distinctly  unfavorable  for  the  development  of  either  salinity  or 
reaction  currents  of  large  volume  and  appreciable  velocity,  and 


146  CURRENT  ACTION 

that  Mitchell's  work  demonstrated  the  tidal  origin  of  the  prin- 
cipal movement.  The  fact  that  a  certain  current  flows  landward 
along  the  bottom  of  a  river  channel  and  consists  of  heavier,  more 
saline  water  than  is  found  above  it,  does  not  mean  that  such 
current  is  caused  by  either  the  dynamic  force  of  the  river  current 
or  the  difference  in  specific  gravity  between  salt  and  fresh  water. 

Another  source  of  diflaculty  in  interpreting  current  movements 
arises  from  the  fact  that  the  currents  usually  observed  are  not 
always  the  ones  which  do  the  most  work.  Thus,  the  prevailing 
winds  may  cause  almost  continuous  but  weak  wind  currents 
and  wave  currents  in  one  direction,  whereas  the  greatest  storms 
may  cause  short-lived  but  remarkal^ly  vigorous  wind  and  wave 
currents  in  the  reverse  direction.  More  material  may  be  moved, 
and  moved  a  greater  distance,  by  the  latter  currents  than  by 
the  more  continuous  weaker  ones.  Hence,  the  direction  for  the 
dominant  transportation  of  beach  material  is  contrary  to  the 
prevailing  currents.  It  has  happened  that  in  such  a  case  one 
observer  erroneously  concluded  that  wave  and  wind  currents  had 
nothing  to  do  with  the  distribution  of  the  beach  material;  while 
another  assumed  that  the  material  must  move  with  these  pre- 
vailing currents,  and  accordingly  developed  erroneous  theories 
regarding  the  laws  of  shingle  transportation. 

A  further  cause  of  misunderstanding  is  the  long  time  which 
waves  and  currents  have  taken  to  produce  certain  effects  ob- 
served along  the  coast.  To  the  geologist,  who  is  familiar  with  the 
slow  operation  of  the  forces  of  nature,  it  seems  that  waves,  at 
least,  work  with  comparative  rapidity.  But  the  ordinary  ob- 
server, and  even  the  skilled  engineer,  may  find  it  chfl[icult  to 
attribute  the  vast  accumulations  of  sand  and  shingle  on  our 
coasts  to  forces  which  seem  to  him  almost  impotent  in  comparison 
with  the  great  work  accomplished.  Such  is  the  view  repeatedly 
expressed  by  Wheeler  in  his  volume  on  "  The  Seacoast."  In 
the  opinion  of  this  eminent  engineer,  ''  a  careful  consideration 
of  all  the  circumstances  that  attach  to  beaches  can  only  lead  to 
the  conclusion  that  the  results  which  have  been  attained  must 
be  due  to  other  and  mightier  forces  than  those  now  in  existence." 
''It  is  certain  that  the  enormous  mass  of  sand,  which  now  covers 
the  littoral  of  the  sea  and  the  beds  of  estuaries,  cannot  have 
been  deposited  by  existing  agencies."  "  The  enormous  accumu- 
lation of  shingle  known  as  the  Chesil  Bank  .  .  .  must  have  been 


COMPLEXITIES  OF  CURRENT  ACTION  147 

accomplished  under  conditions  very  different  to  those  which  now 
exist  "1^^.  The  geologist,  on  the  other  hand,  recognizes  in  these 
extensive  shore  deposits  the  effects  of  ordinary  forces  of  nature 
continued  for  a  very  long  period  of  time.  There  is  nothing  in 
the  deposits  described  by  Wheeler  to  excite  wonder,  except  their 
extent;  and  large  deposits  may  be  made  by  ordinary  forces 
working  a  long  time  as  well  as  by  extraordinary  forces  working 
a  short  time.  I  have  examined  some  of  the  largest  beach  ac- 
cumulations on  the  English  and  other  European  coasts,  as  well 
as  those  on  the  Atlantic  coast  of  the  United  States,  and  see  no 
reason  to  doubt  that  they  have  been  produced  by  the  same  waves 
and  currents  which  are  still  at  work  upon  them. 

Conclusions.  —  In  the  preceding  paragraphs  I  have  endeavored 
to  give  the  reader  some  idea  of  the  serious  difficulties  which  con- 
front the  student  of  shore  processes.  It  must  be  confessed, 
however,  that  it  is  much  easier  to  describe  the  complexities  of 
currents,  and  to  point  out  the  mistakes  which  are  frequently 
made  in  interpreting  them,  than  it  is  to  solve  those  complexities 
in  a  given  case  and  present  a  discussion  which  is  so  conclusive  as 
not  to  be  open  to  criticism.  Nevertheless,  it  was  essential  that 
we  should  enter  upon  our  treatment  of  shoreline  forms  with  a 
broad  view  of  the  problems  connected  with  wave  and  current 
action,  and  with  some  appreciation  of  the  variety  of  the  forces 
which  operate  at  the  shore  in  different  places  and  at  different 
times.  We  are  now  prepared  to  consider  the  development  of 
shorelines  more  intelligently,  even  if  we  are  not  prepared  to 
assert  with  positiveness  the  precise  part  played  by  different  cur- 
rents in  shaping  each  portion  of  any  given  shore. 

The  time  will  come  when  our  present  limited  knowledge  of 
both  wave  and  current  action  will  be  enormously  extended  by 
means  of  improved  mechanical  appliances.  The  movements  of 
debris  upon  the  bottom  at  considerable  depths  during  wave  ac- 
tion, concerning  which  we  can  only  theorize  at  present,  will  be 
actually  observed  by  special  electrical  apparatus.  Wave  cur- 
rents and  currents  of  other  types  will  be  studied  by  observing  the 
exact  movements  of  debris  under  their  control.  Limited  areas 
of  the  coastal  waters  will  be  exhaustively  studied,  every  detail 
of  the  currents  analyzed  with  care  under  varying  conditions,  and 
the  movements  of  debris  determined  with  far  greater  precision 
than  is  now  possible.     While  shoreline  problems  will  never  be 


148  CURRENT  ACTION 

simple,  the  researches  of  the  future  will  yield  a  bod}'-  of  facts 
which  will  enable  the  geologist  and  engineer  of  some  coming 
generation  to  predict  shore  changes  and  plan  harbor  and  coast 
defenses  with  an  assurance  which  will  contradict  the  assertion 
of  the  present  maritime  engineer,  that  the  forces  operating  at 
the  shore  are  among  the  forces  of  nature,  "  which  are  subject  to 
no  calculation."  In  the  meantime  we  may  take  some  satis- 
faction from  the  fact,  which  will  presently  appear,  that  a  great 
deal  may  be  learned  about  current  action  by  studying  the  forms 
of  beaches,  Since  these  often  provide  a  more  reliable  indication 
of  the  dominant  currents  in  a  gi  /en  locality  than  do  any  direct 
observations  feasible  at  the  present  time. 

RESUME 

We  have  reviewed  the  essential  characteristics  of  the  more 
important  types  of  currents,  and  gained  some  idea  of  their  rela- 
tive strength,  and  comparative  importance  in  shore  processes. 
It  appears  that  a  great  variety  of  wave  currents  operate  in  a 
most  complicated  and  irregular  manner,  sorting  and  transport- 
ing debris  in  shallow  water  and  on  the  beach  in  different  ways 
depending  on  differences  in  outline  of  shore,  angle  of  offshore 
slope,  angle  of  wave  approach,  size  of  waves,  kind  of  waves, 
and  other  factors.  Tidal  currents  are  scarcely  less  complicated, 
although  developed  on  a  much  larger  scale,  and  therefore  more 
easily  studied.  Seiche  currents,  wind  currents,  planetary  cur- 
rents, pressure  currents,  convection  currents,  salinity  currents, 
river  currents,  reaction  currents,  eddy  currents,  and  hydraulic 
currents  have  all  been  considered;  and  we  have  found  that 
some  of  them  have  a  small  degree  of  local  importance  only,  while 
others  are  of  wide-spread  occurrence,  or  have  a  volume  and 
strength  which  make  them  of  very  great  significance.  These 
currents  are  deflected  from  their  initial  courses  by  the  earth's 
rotation;  they  combine  with  each  other  or  counteract  each 
other  in  the  most  complicated  ways;  they  are  not  infrequently 
wrongly  identified,  and  their  manner  of  working  and  relative 
importance  are  often  matters  of  dispute.  Some  knowledge  of 
their  behavior  is  nevertheless  essential  to  an  understanding  of 
shore  forms,  and  we  may  in  turn  expect  to  gain  further  know- 
ledge of  the  currents  themselves  when  we  study  the  forms  they 
have  helped  to  produce. 


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1895.     London,  1906. 

116.  Ekman,  F.  L.     On  the  General  Causes  of  the  Ocean  Currents.     Nova 

Acta  Regise  Societatis  Scientiarum  Upsaliensis.     Serie  3,  X,  11,  1876. 

117.  Ibid.,  p.  11. 

118.  Pettersson,    Otto.     On   the   Influence   of   Ice-Melting  upon  Oceanic 

Circulation.     Svenska  Hydrografisk  Biologiska  Kommissionens,  Skrif- 
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120.  Barnes,  H.  T.     Report  on  the  Influence  of  Icebergs  and  Land  on  the 

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122.  Harris,  R.  A.     Manual  of  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907.     Appendix  No.  6,  p.  441,  1907. 

123.  Ibid.,  p.  441. 

124.  Dawson,  W.  Bell.     Report  of  Progress  for  the  Year  1894  in  the  Sur- 

vey of  Tides  and  Currents  in  Canadian  Waters.     Proc.  Roy.  Soc. 
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125.  Grabau,  a.  VV.     Principles  of  Str  tigraphy,  p.  241,  New  York,  1913. 

126.  Otto,  Theodor.     Der  Darss  and  Zingst.     Jahresb.  der  Geogr.  Gesells  zu 

Greifswald.     XIII,  363,  1913. 

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Acta  Regiae  Socistatis  Scientiarum  Upsaliensis.     Serie  3,  X,  29,  1876. 

128.  Pettersson,   Otto.     On  the   Influence  of   Ice-melting  upon  Oceanic 

Circulation.     Svenska  Hydrografisk  Biologiska  Kommissionens  Skrif- 
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129.  Bjerknes,  V.  and  Sandstrom,  J.  W.     Uber  die  Darstellung  des  Hydro- 

graphischen    Beobachtungsmateriales    durch    Schnitte,    pp.    10,    18, 
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130.  Grabau,  A.  W.     Principles  of  Stratigraphy,  p.  240,  New  York,  1913. 

131.  Fischer,     Theobald.     Zur     Entwickelungs-Geschichte     der     Kiisten. 

Petermanns  Geographische  Mitteilungen.     XXXI,  415,  1885. 

132.  Maury,  M.  F.     The  Physical  Geography  of  the  Sea  and  its  Meteor- 

ology, p.  184,  London,  18S1. 

133.  Harris,  R.  A.     Manual  of  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907.     Appendix  No.  6,  p.  441,  1907. 

134.  Murray,  John  and  Hjort,  Johan,  d  al.     The  Depths  of  the  Ocean, 

pp.  285-287,  London,  1912. 

135.  Lindenkohl,  A.     Oceanography.     Encyclopedia  Americana,  1904. 
Harris,  R.  A.     Manual  <  f  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907.     Appendix  Xo.  6,  p.  441,  1907. 

136.  Murray,  John  and  H,jort,  Johan,  d  al.     The  Depths  of  the  Ocean, 

p.  289,  London,  1912. 

137.  Reade,  T.  Mellard.     Tidal  Action  as  an  Agent  of  Geological  Change. 

Philosophir-al  Magazine,  XXV,  342,  1888. 

138.  Ibid.,  p.  342. 

139.  BucHAN,    Alexander.     Specific    Gravities    and    Oceanic    Circulation. 

Trans.  Roy.  Soc.  Edinburgh.     XXXVIII,  327,  1897. 

140.  Harris,  R.  A.     Manual  of  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907.     Appendix  No.  6,  p.  442,  1907. 


156  CURRENT  ACTION 

Grabau,  a.  W.     Principles  of  Stratigraphy,  p.  241,  New  York,  1913. 
EJtUMMEL,     Otto.     Handbuch    der    Ozeanographie.     II.    Die    Bewe- 
gungsformen  des  Meeres,  p.  686,  Stuttgart,  1911. 

141.  Dall,  W.  H.     Harbors  of  Alaska  and  the  Tides  and  Currents  in  their 

Vicinity.     U.  S.  Coast  Surv.  Rept.  for  1872,  p.  190,  1875. 

142.  Geikie,  A.     Textbook  of  Geology.     4th  Edition,  p.  515,  London,  1903. 

143.  Le  Conte.     Elements  of  Geology.     2nd  Ed.,  p.  40,  1882. 
Branner,  J.  C.     The  Poror6ca,  or  Bore,  of  the  Amazon.     Science,  IV, 

492,  1884. 

144.  Ek\l\n,  F.  L.     On  the  General  Causes  of  the  Ocean  Currents.     Nova 

Acta  Regiae  Societatis  Scientiarum  Upsaliensis,  Serie  3,  X,  16-37,  1876. 

145.  Cornish,  Vaughan.      On  Sea  Beaches  and  Sand  Banks.      Geog.  Jour., 

XI,  529,  London,  1898. 

146.  Ekman,  F.  L.     On  the  General  Causes  of  the  Ocean  Currents.     Nova 

Acta  Regia)  Societatis  Scientiarum  Upsaliensis.     Serie  3,  X,  23,  1876. 

147.  Cronander,  a.  W.     On  the  Laws  of  Movement  of  Sea  Currents  and 

Rivers,  pp.  48-52,  Norrkoping,  1898. 

148.  Ekman,  V.  W.     Ein  Beitrag  zur  Erklarung  und  Berechnung  des  Strom- 

verlaufs  an  Flussmiindungen.     Kongl.  Vetenskaps-Akademiens  Ford- 
handlingar,  pp.  479-507,  1899. 

149.  Ibid.,  p.  501. 

150.  Buchanan,   J.   Y.     On  the   Land  Slopes  Separating  Continents   and 

Ocean  Basins.     Collected  Papers.     I,  No.  39,  31  pp.,  Cambridge,  1913. 

151.  Harris,  R.  A.     Manual  of  Tides.     Part  V.     U.  S.  Coast  Surv.  Rept. 

for  1907.     Appendix  No.  6,  p.  356,  1907. 
Bache,  a.  D.     On  the  Tidal  Currents  of  New  York  Harbor  near  Sandy 
Hook.     U.  S.  Coast  Surv.,  Rept.  for  1858,  p.  200,  1859. 

152.  Gulliver,    F.    P.      Cuspate    Forelands.     Bull.    Geol.   Soc.   Am.    VII, 

413-417,  1896. 

153.  Abbe,  Cleveland,  Jr.     Remarks  on  the  Cuspate  Capes  of  the  Caro- 

hna  Coast.     Proc.  Bost.  Soc.  Nat.  Hist.     XXVI,  496-497,  1895. 

154.  Vaughan,  T.  W.     A  Contribution  to  the  Geological  History  of  the 

Floridian  Plateau.     Carnegie  Institution,  Papers  from  the  Tortugas 

Labt)ratory.     IV,  142,  1910. 
Gulliver,  F.  P.     ShoreUne  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  180,  1899. 
Hunt,  E.  B.     On  the  Origin,  Growth,  Substructure,  and  Chronology 

of  the  Florida  Reef.     Am.  Jour.  Sci.     2nd  Ser.,  XXXV,  198,  1863. 
Agassiz,  Alexander.     The  Tortugas  and  Florida  Reefs.     Am.  Acad. 

Mem.     XI,  108,  1883. 

155.  Perkins,  F.  W.     [On  the  origin  of  the  Florida  counter  current.]     Per- 

sonal communication,  1914. 

156.  Murray,  John  and  Hjort,  Johan,  et  al.     The  Depths  of  the  Ocean, 

p.  270,  London,  1912. 

157.  Dall,  W.  H.     Harbors  of  Alaska  and  the  Tides  and  Currents  in  their 

Vicinity.     U.  S.  Coast  Surv.,  Rept.  for  1872,  p.  188,  1875. 

158.  Harris,  R.  A.     Manual  of  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907,  Appendix  No.  6,  p.  442,  1907. 


REFERENCES  157 

159.  Mill,    H.    R.     The    Clyde   Sea-area.     Trans.    Roy.    Soc.    Edinburgh. 

XXXVI,  653,  1892. 

160.  Ekman,  F.  L.     On  the  General  Causes  of  the  Ocean  Currents.     Xova 

Acta  Regiae  Societatis  Scientiarum  Upsaliensis.  Serie  3,  X,  45-47, 
1876. 

161.  BucHAN,   Alexander.      Specific    Gravities    and   Oceanic    Circulation. 

Trans.  Roy.  Soc.  Edinburgh.     XXXVIII,  317-342,  1897. 
Ekman,  F.  L.     On  the  General  Causes  of  the  Ocean  Currents.     Nova 
Acta  Regiae  Societatis  Scientiarum  Upsaliensis.    Serie  3,  X,  37-45,  1876. 

162.  Parsons,  H.  de  B.     Tidal  Phenomena  in  the  Harbor  of  New  York. 

Proc.  Am.  Soc.  Civ.  Eng.     XXXIX,  659-670,  1913. 

163.  Ekman,  F.  L.     On  the  General  Causes  of  the  Ocean  Currents.     Nova 

Acta  Regia}  Societatis  Scientiarum  Upsaliensis.     Serie  3,  X,  32,  1876. 

164.  Hunt,  A.  R.    On  the  Action  of  Waves  on  Sea  Beaches  and  Sea  Bottoms. 

Proc.  Roy.  Dublin  Soc,  N.  S.     IV,  274,  1884. 

165.  Cronander,  a.  W.     On  the  Laws  of  Movement  of  Sea  Currents  and 

Rivers,  pp.  15,  23,  31,  Norrkoping,  1898. 

166.  Harris,  R.  A.     Manual  of  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907,  Appendix  No.  6,  p.  356,  1907. 

167.  Otto,  Theodor.     Der  Darss  und  Zingst.     Jahresb.  der  Geogr.  Ges.  zu 

Greifswald.     XIII,  362,  1913. 

168.  Wheeler,  W.  H.     The  Sea  Coast:    Destruction:    Littoral  Drift:    Pro- 

tection, p.  16,  London,  1902. 

169.  Hunt,  A.  R.     On  the  Action  of  Waves  on  Sea  Beaches  and  Sea  Bottoms. 

Proc.  Roy.  Dublin  Soc,  N.  S.     IV,  241,  1884. 

170.  CooDE,  John.     Description  of  the  Chesil  Bank,  with  Remarks  upon  its 

Origin,  the  Causes  which  have  Contributed  to  its  Formation,  and 
upon  the  Movement  of  Shingle  Generally.  Min.  Proc.  Inst.  Civ.  Eng. 
XII,  593,  1853. 

171.  Prestwich,  Joseph.     On  the  Origin  of  the  Chesil  Bank,  and  on  the 

Relation  of  the  Existing  Beaches  to  Past  Geological  Changes  Inde- 
pendent of  the  Present  Coast  Action.  Min.  Proc  Inst.  Civ.  Eng. 
XL,  101,  1875. 

172.  Ibid.,  p.  81. 

173.  Ihid.,  p.  78. 

174.  Palmer,  H.  R.     Observations  on  the  Motions  of  Shingle  Beaches.    Phil. 

Trans,  of  the  Royal  Society.     CXXIV,  Pt.  I,  569,  1834. 

175.  Prestwich,  Joseph.     On  the  Origin  of  the  Chesil  Bank,  and  on  the 

Relation  of  the  Existing  Beaches  to  Past  Geological  Changes  Inde- 
pendent of  the  Present  Coast  Action.  Min.  Proc.  Inst.  Civ.  Eng. 
XL,  89,  1875 

176.  lUd.,  p.  89. 

177.  Geikie,  a.     Textbook  of  Geology.     4th  Edition,  I,  576,  1903. 

178.  CooDE,  John.     Description  of  the  Chesil  Bank,  with  Remarks  upon 

its  Origin,  the  Causes  which  have  Contributed  to  its  Formation,  and 
upon  the  Movement  of  Shingle  Generally.  Min.  Proc.  Inst.  Civ. 
Eng.     XII,  540,  1853. 

179.  Matthews,  E.  R.     Coast  Erosion  and  Protection,  p.  6,  London,  1913. 


158  CURRENT  ACTION 

180.  Shield,   William.     Principles  and  Practice  of  Harbor  Construction, 

p.  41,  London,  1895. 

181.  IviXAHAN,  G.  H.     The  Travelling  of  Sea  Beaches.     }^Iin.  Proc.  Inst. 

Civ.  Eng.     LVIII,  281,  1879. 

182.  Palmer,    H.    R.     Observations   on  the   Motions   of   Shingle  Beaches. 

Phil.  Trans,  of  the  Royal  Society.     CXXIV,  Pt.  I,  571,  1834. 

183.  CooDE,  John.     Description  of  the  Chesil  Bank,  with  Remarks  upon 

its  Origin,  the  Causes  which  have  Contributed  to  its  Formation,  and 
upon  the  Movement  of  Shingle  Generally.  Min.  Proc.  Inst.  Civ. 
Eng.     XII,  540,  1853. 

184.  Reade,  T.  Mellard.     Tidal  Action  as  an  Agent  of  Geological  Change. 

Philosophical  Magazine.     XXV,  342,  1888. 
Dana,  J.  D.     Manual  of  Geology.     4th  Edition,  p.  229,  1895. 

185.  Mitchell,  Henry.     The  Under-run  of  the  Hudson  River.     U.  S.  Coast 

Surv.  Rept.  for  1887,  p.  301,  1889. 

186.  Harris,  R.  A.     Manual  of  Tides,  Part  V.     U.  S.  Coast  Surv.  Rept.  for 

1907,  Appendix  No.  6,  p.  442,  1907. 

187.  Wheeler,  W.  H.     The  Sea  Coast:    Destruction:    Littoral  Drift:    Pro- 

tection, pp.  23-25,  London,  1902. 


CHAPTER  IV 
TERMINOLOGY  AND  CLASSIFICATION  OF  SHORES 

Advance  Summary.  —  Before  undertaking  a  systematic  dis- 
cussion of  shoreline  development  it  is  important  to  adopt  a 
satisfactory  terminology  for  the  topographic  elements  of  shores, 
and  to  agree  upon  a  classification  of  shorelines  which  shall  serve 
as  a  guide  throughout  this  treatment.  These  are  the  two  tasks 
attempted  in  the  present  chapter.  With  the  aid  of  explanatory 
diagrams  the  terminology  used  in  this  volume  is  made  clear. 
The  discussion  of  shore  terminology  leads  inevitably  to  a  con- 
sideration of  the  terminology  of  peneplanes  of  marine  and  other 
origins,  and  space  is  given  to  an  inquiry  into  the  proper  signifi- 
cance of  the  terms  plains,  planes,  and  peneplanes.  The  classifi- 
cation of  shorelines  is  next  essayed.  After  a  brief  review  of 
previous  methods  of  classification,  a  genetic  scheme  is  adopted 
in  which  four  primary  types  of  shorehne  are  recognized.  These 
are  described,  their  chief  subdivisions  named,  and,  where  cir- 
cumstances make  it  advisable,  discussed  at  some  length.  It  is 
further  pointed  out  that  each  class  or  sub-class  of  shorelines 
passes  through  its  appropriate  young,  mature  and  old  sequential 
stages  of  development. 

Terminology  of  Shores.  —  The  line  where  land  and  water  meet 
has  been  called  the  shoreline,  the  strandline,  the  coast  line,  and 
the  water  hne.  The  terms  shore,  beach,  strand,  and  coast  are 
also  loosely  used  with  varying  significance  by  different  writers. 
"  Shore  "  is  defined  by  Gulliver^  as  the  water  area  immediately 
seaward  from  the  shoreline;  by  modern  legal  authorities,  as  the 
space  between  low  water  and  high  water;  and  by  Wheeler^,  as  the 
land  area  immediately  above  high  water.  "  Beach  "  is  sometimes 
used  to  denote  the  zone  between  low  water  and  high  water,  or 
to  denote  the  debris  found  between  those  limits,  while  others 
regard  it  as  extending  some  distance  below  low  water.  ''  Coast  " 
may  mean  the  narrow  strip  immediately  landward  from  the 
shoreline^,  or  it  may  imply  a  much  broader  zone  extending  some 
distance  inland.     RatzeP  discusses  at  some  length  the  varying 

159 


160       TERMINOLOGY   AND   CLASSIFICATION   OF  SHORES 

significance  attached  to  the  word  coast  by  different  writers.  Evi- 
dently there  is  a  variety  of  usage  in  naming  shore  features,  even 
among  scientific  workers.  It  is  essential,  therefore,  that  we  adopt 
a  terminology  to  be  used  throughout  this  discussion,  and  an 
effort  will  be  made  to  secure  the  required  precision  with  the  least 
possible  departure  from  common  usages. 

At  the  margin  of  the  sea  there  are  typically  found  three  or  four 
distinct  zones,  each  of  which  is  characterized  by  certain  peculiar 
forms  due  to  deposition  or  erosion.  The  zones,  the  erosion  fea- 
tures, and  the  features  due  to  deposition  must  each  be  clearly 
distinguished  and  receive  appropriate  names  (Figs.  22  and  23). 
The  most  important  of  the  four  zones  extends  from  low  water 
mark   to  the  base  of  the  cliff,  whether  large   or  small,  which 


Fig.  22.  —  Elements  of  the  shore  zones  during  an  early  stage  of  development. 

usually  marks  the  landward  limit  of  effective  wave  action.  This 
is  the  zone  over  which  the  water  line,  the  line  of  contact  between 
land  and  sea,  migrates;  and  it  will  here  be  called  the  shore.  It 
is,  indeed,  the  zone  most  commonly  referred  to  when  the  word 
shore  is  employed  in  ordinary  speech,  and  is  likewise  the  zone 
defined  as  the  shore  in  Roman  law. 

Landward  from  the  shore  is  a  much  broader  zone  of  indeter- 
minate width,  which  will  here  be  called  the  coast.  While  some 
may  have  more  or  less  consciously  included  the  shore  when 
referring  to  a  coast,  it  is  also  quite  common  to  exclude  it,  by 
implication  at  least,  as  when  one  says  that  a  coast  terminates 
in  a  series  of  ragged  cliffs.  Indeed,  the  narrow  shore  zone  is 
probably  seldom  thought  of  when  a  coast  is  referred  to,  and  it 
will  conduce  to  clearness  if  we  restrict  the  terms  shore  and  coast 
to  the  two  independent  zones.  The  line  which  forms  the  bound- 
ary between  these  zones  is  the  coast  line,  and  it  marks  the  sea- 
ward limit  of  the  permanently  exposed  coast.     In  a  correspond- 


TERMINOLOGY  OF  SHORES  161 

ing  manner  the  low  tide  shoreline  marks  the  seaward  limit  of  the 
intermittently  exposed  shore.  The  position  of  the  water  line  at 
high  tide  marks  the  high  tide  shoreline.  When  the  term  "  shore- 
line "  alone  is  used  in  the  text,  low  tide  shoreline  is  to  be  under- 
stood. 

Seaward  from  the  low  tide  shoreline  is  a  narrow  zone  per- 
manently covered  by  water,  over  which  the  beach  sands  and 
gravels  actively  oscillate  with  changing  wave  conditions.     Al- 
though of  great  importance  to  the  student  of  shores,  this  zone 
has  no  suitable  name.     Gulliver'^  recognized  this  difficulty,  and 
proposed  to  call  the  zone  the  "shore";    but  his  suggestion  is 
hardly  acceptable  in  view  of  the  fact  that  "  shore  "  is  almost 
universally  applied  to  some  part  of  the  land  area  inside  the  low 
water  mark.     Gulliver,  furthermore,  recognized  only  two  zones, 
the   coast   and   the   shore.     "  Inshore "   is   sometimes   used   as 
opposed  to  "  offshore,"  but  is  quite  uniformly  applied  to  a  broader 
zone  than  that  now  under  consideration;    as,  for  example,  in 
the  expression   "  inshore  fishing,"   which   may  refer  to   fishing 
carried  on  from  one  to  three  miles  from  the  land.     The  term 
"  shore  face  "  as  used  by  Barrel^  in  his  discussion   of    deltas 
applies  to  much  of  the  zone  here  in  question,  and  after  confer- 
ence with  that  author  I  have  decided  to  adopt  his  term,  writing 
it  as  one  word,  shoreface,  and  redefining  it  as  the  zone  between 
the  low  tide  shoreline  and  the  beginning  of  the  more  nearly 
horizontal  surface  included  in  the  zone  next  defined.     Extending 
from  the  outer  margin  of  the  rather  steeply  sloping  shoreface  to 
the  edge  of  the  continental  shelf  is  a  comparatively  flat  zone 
of  variable  width  which  will  be  called  the  offshore  belt,  or  simply 
the  offshore  (Fig.  23).     This  is  the  zone  commonly  referred  to 
in  such   expressions   as    "  offshore    sediments,"    and    "  offshore 
deposition." 

The  shore  is  sub-divided  into  two  minor  zones.  One  of  these 
lies  between  the  ordinary  high  and  low  water  marks,  and  is 
daily  traversed  by  the  oscillating  water  line  as  the  tides  rise  and 
fall.  This  zone  is  already  well  known  as  the  foreshore.  Back  of 
it  is  the  portion  of  the  shore  covered  by  water  during  exceptional 
storms  only,  which  I  propose  to  call  the  hackshore. 

The  wave-erosion  features  associated  with  the  coast,  shore, 
shoreface,  and  offshore,  are  three  in  number.  At  the  seaward 
edge  of  the  coast  is  the  wave  cut  cliff,  which  varies  in  magnitude 


162       TERMINOLOGY  AND  CLASSIFICATION  OF  SHORES 


f 


from  an  inconspicuous  slope  at  the 
margin  of  a  low  coastal  plain  or  in  the 
side  of  a  sand  dune,  to  an  escarpment 
hundreds  of  feet  in  height.  In  front  of 
it,  and  occupying  all  of  the  shore  zone 
and  part  or  all  of  the  shoreface  is  the 
wave  cut  hench,  a  sloping  erosion  plane 
inclined  seaward.  The  bench  may  end 
aljruptly  at  the  top  of  a  steeper  slope 
representing  part  of  the  original  surface 
of  the  sea-bottom  (Fig.  22);  or  it  may 
gradually  decrease  in  slope  until  it 
merges  imperceptibl}^  into  the  more  ex- 
tensive, nearl}^  horizontal  plane  pro- 
duced by  long  continued  wave  erosion, 
which  is  commonly  called  the  abrasion 
platform  (Fig.  23). 

There  are  three  characteristic  de- 
posits which  rest  upon  the  wave  cul^ 
l)ench  and  the  slopes  which  lie  to  sea- 
ward of  it.  Most  important  of  these 
is  the  deposit  of  material  which  is  in 
more  or  less  active  transit,  along  shore 
or  on-and-off  shore,  and  which  will  be 
called  the  beach.  Gilbert^  defined  the 
beach  as  "  the  zone  occupied  by  the 
shore  drift  in  transit."  It  seems  to 
the  present  writer  that  the  descriptions 
of  beaches  given  by  that  careful  and 
scholarly  investigator  of  the  topo- 
graphic features  of  lake  shores  really 
deal  with  the  deposits,  and  not  with 
the  zone  in  which  those  deposits  occur; 
and  what  I  have  suggested  is  therefore 
a  difference  in  phraseology  rather  than 
a  real  difference  of  interpretation.  It 
would  be  unfortunate  to  have  two 
names  for  the  same  zone,  and  none  for 
the  deposit  which  may  or  may  not 
occur  in  that  zone,  as  would   be  the 


TERMINOLOGY  OF  SHORES  163 

case  if  we  accepted  Gilbert's  definition  literally;  for  the  zone 
over  which  material  is  in  transit  certainly  includes  the  shore, 
and  the  transit  of  the  material  may  either  be  so  slow  that  some 
of  it  accumulates  to  form  a  deposit  within  the  shore  zone,  or  so 
rapid  that  bare  rock  is  continuously  exposed  there  (Fig.  22). 
Furthermore,  the  term  "  beach,"  as  originally  used,  referred  to 
the  "shingle"  or  pebbles  found  on  many  of  the  English  shores, 
and  is  employed  in  this  sense  in  some  parts  of  England  to  the 
present  day. 

Near  the  edge  of  the  wave  cut  bench  a  portion  of  the  beach  is 
often  fashioned  into  a  terrace  which  is  for  a  time  progressively 
built  out  into  the  deeper  water,  only  to  be  later  modified  or 
destroyed  during  heavy  storms.  This  wave-built  terrace  may 
be  called  the  shoreface  terrace  to  distinguish  it  from  a  series  of 
terraces  caused  by  the  action  of  storm  waves  on  the  upper  part 
of  the  beach,  and  which  we  will  call  the  backshore  terraces  (Fig.  23). 
As  the  abrasion  platform  is  developed  it  may  be  covered  with  a 
thin  deposit  of  material  in  slow  transit,  which  constitutes  the 
veneer.  At  the  outer  margin  of  the  abrasion  platform  there  accu- 
mulates an  extensive  deposit  of  the  material  which  has  been 
moved  across  the  platform  to  a  more  permanent  resting  place  in 
the  deeper,  quieter  water  beyond.  This  we  will  call  the  conti- 
nental terrace;  together  with  the  abrasion  platform  it  makes  the 
continental  shelf. 

It  will  be  shown  in  the  following  chapter  that  if  a  land  mass 
stands  still  long  enough,  the  waves  will  reduce  it  to  an  ultimate 
abrasion  platform;  and  this  is  true,  no  matter  how  great  may 
have  been  the  original  extent  of  the  land.  The  final  stage  of 
shore  development  will  witness  the  extinction  of  all  of  the  above 
mentioned  features,  except  only  the  abrasion  platform  and  the 
continental  terrace.  Even  the  veneer  may  be  removed  and 
the  bare  rock  surface  of  the  platform  exposed.  Should  an  uplift 
raise  the  platform  high  above  sealevel,  stream  erosion  might 
dissect  the  new  land  area  until  only  remnants  of  the  former 
smooth  erosion  surface  would  be  left.  There  are  many  dis- 
sected erosion  surfaces  in  the  world,  some  of  which  probably 
represent  uplifted  abrasion  platforms.  If  the  platform  were 
reduced  pra.ctically  to  a  plane  surface  before  uplift,  the  uplifted 
surface  may  be  called  a  plane  of  marine  denudation,  or  simply 
a  marine  plane.     On  the  other  hand,  if  wave  erosion  had  not 


164       TERMINOLOGY   AND  CLASSIFICATION   OF  SHORES 

yet  succeeded  in  perfecting  a  smooth  plane  when  uplift  raised 
the  platform  above  the  reach  of  the  waves  to  form  a  land  area, 
the  uplifted  surface  should  be  called  a  marine  peneplane.  These 
last  two  terms  involve  a  slight  revision  of  former  usage,  which 
cannot  be  appreciated  without  some  consideration  of  the  ter- 
minology of  erosion  planes  in  general.  We  may  therefore  turn 
our  attention  for  a  few  moments  to  this  broader  subject. 

Plains,  Planes,  and  Peneplanes.  —  In  an  erosion  cycle  of  any 
kind  a  land  mass  will  in  time  be  worn  down  to  a  smooth  surface, 
providing  the  process  of  erosion  is  not  interrupted.  Long  con- 
tinued wave  erosion  reduces  the  land  to  a  plane  below  sealevel; 
long  continued  stream  erosion,  to  a  plane  at  sealevel;  and  long 
continued  wind  erosion,  to  a  surface  at  some  elevation  above 
the  sea  which  will  then  be  progressively  lowered  until  sealevel  is 
reached.  Long  continued  glacial  erosion  may  possibly  produce 
a  plane  or  a  concave  surface,  either  above  or  below  sealevel; 
but  the  cycle  of  glacial  erosion  is  not  so  well  understood  as  are 
the  other  three  cycles  mentioned. 

The  work  of  any  erosive  force  may  be  interrupted  after  the 
land  has  been  worn  down  to  a  gently  undulating  surface  of  low 
relief,  but  before  complete  planation  has  been  accomplished. 
We  must  recognize,  therefore,  not  only  the  plane  surfaces  of 
ultimate  erosion,  but  imperfect,  "  almost-plane  "  surfaces  which 
characterize  the  penultimate  stages  of  the  several  cycles.  Theo- 
retically, at  least,  there  may  be  three,  or  possibly  four,  planes 
of  erosion,  with  a  corresponding  number  of  almost-plane  sur- 
faces of  uncompleted  erosion,  due  to  the  action  of  rivers,  waves, 
winds,  and  possibly  glaciers. 

Davis  has  given  the  name  "  peneplain  "  to  the  almost-plane 
surface  of  uncompleted  fluvial  denudation.  The  other  almost- 
plane  surfaces  remain  unnamed,  except  that  such  terms  as  "  plain 
of  marine  denudation,"  "  plain  of  marine  abrasion,"  and  "  plain 
of  aeolian  erosion  "  have  been  applied  to  erosion  surfaces, 
usually  without  regard  to  the  question  whether  they  were  really 
plane  surfaces,  or  only  surfaces  of  moderate  relief.  An  exception 
to  the  foregoing  statement  is  perhaps  to  be  found  in  Gulliver's 
valuable  paper  on  "  Shoreline  Topography,"  where  he  seems  to 
call  the  almost  plane  surface  of  marine  erosion  a  "  submarine 
platform,"  although  he  makes  it  identical  with  "  plain  of  marine 
denudation,"  and  would  therefore  seem  to  have  no  name  for  a 


PLAINS,  PLANES,   AND   PENEPLANES  165 

perfectly  plane  surface  of  marine  denudation^.  It  does  not  seem 
advisable  to  apply  the  adjective  "  submarine  "  to  a  surface  which 
is  today  far  above  sealevel,  so  some  other  name  should  be 
sought  for  up-lifted  surfaces  of  marine  erosion. 

It  is  coming  to  be  recognized  that  some  of  the  so-called  pene- 
plains are  more  probably  almost-plane  surfaces  of  marine  erosion, 
as  was,  indeed,  the  opinion  of  the  earlier  geologists.  BarrelP 
has  even  questioned  the  subaerial  origin  of  the  New  England 
uplands,  supposedly  a  typical  peneplain,  and  the  one  above  which 
rises  the  mountain  selected  as  the  type  "  monadnock."  Should 
his  conclusions  prove  correct,  and  apply  to  the  portion  of  the 
supposed  peneplain  in  southern  New  Hampshire,  then  not  only 
would  the  upland  cease  to  merit  the  term  "  peneplain,"  but  Mt. 
Monadnock  would  no  longer  be  a  "  monadnock,"  as  that  term 
is  generally  defined.  We  would  have  to  change  the  definition 
of  monadnock,  or  invent  a  new  name  for  the  topographic  feature 
of  which  it  is  an  example. 

A  further,  difficulty  arises  from  the  fact  that  the  word  "  plain  " 
is  used  for  two  such  very  different  conceptions  as  a  plane  of 
ultimate  erosion,  and  a  series  of  low-lying  horizontal  sediments 
which  may  be  dissected  into  hills  and  valleys  by  stream  erosion. 
A  peneplain  is  not  "  almost  a  plain  "  of  the  second  type,  but 
is  almost  a  plane  surface  in  the  mathematical  sense  of  the 
term. 

It  appears,  therefore,  that  we  need-  names  for  the  different 
types  of  erosion  surfaces  which  theoretically  may  be  produced 
both  in  the  penultimate  and  the  ultimate  stages  of  erosion; 
that  we  also  need  a  name  for  the  actually  existing  upland  sur- 
faces of  erosion  which  are  so  abundant  in  different  parts  of  the 
world  but  of  whose  origin  we  are  not  yet  certain;  and,  finally, 
that  we  need  a  clearer  distinction  in  the  names  themselves, 
between  '  plains "  of  erosion,  "  plains "  of  deposition,  and 
"  peiieplains."  I  believe  it  is  possible  to  meet  all  these  needs 
without  departing  unduly  from  the  path  of  conservatism;  and 
to  this  end  the  following  usage  will  be  adhered  to  in  future 
pages:  (1)  The  level  erosion  surface  produced  in  the  ultimate 
stage  of  any  cycle  will  be  called  a  plane.  (2)  The  undulating 
erosion  surface  of  moderate  relief  produced  in  the  penultimate 
stage  of  any  cycle  will  be  called  a  peneplane.  (3)  A  low-relief 
region  of  horizontal  rocks  will  be  called  a  plain.     We  may  then 


166       TERMINOLOGY  AND  CLASSIFICATION    OF  SHORES 

recognize  planes  of  fluvial,*  marine,  aeolian,  and  glacial  denuda- 
tion; also  fluvial  peneplanes,  marine  peneplanes,  aeolian  pene- 
planes,  and  glacial  peneplanes.  A  monadnock  may  be  defined 
as  an  erosion  remnant  left  standing  above  a  peneplane  of  any 
origin,  either  because  it  is  composed  of  more  resistant  rock 
or  because  it  has  been  less  exposed  to  the  agents  of  erosion. 
Vogt^"  has  already  used  the  term  "  Monadnock-Berge "  for 
isolated  hills  on  the  uplifted  marine  abrasion  plane  of  northern 
Norway.  It  seems  desirable  to  employ  a  special  term  for  a 
surface  of  marine  denudation  which  is  still  in  its  original  po- 
sition at  or  near  wave  base,  with  the  marine  forces  still  operating 
on  it,  and  for  this  feature  the  name  abrasion  platform  has  al- 
ready been  used.  An  uplifted  abrasion  platform  of  large  areal 
extent  is  a  marine  peneplane  or  a  marine  plane,  according  to  the 
smoothness  of  the  surface  produced  by  wave  erosion. 

The  advantages  of  this  usage  are  obvious.  It  tends  to  sim- 
plify, not  to  complicate  physiographic  terminology.  The  origin 
of  any  of  the  level  or  almost  level  surfaces  here  discussed  is  at 
once  apparent  from  the  spelling.  If  in  describing  a  coastal 
region  one  uses  the  otherwise  non-commital  term  "  marine  plain," 
in  the  manner  here  suggested,  the  reader  knows  at  once  that  a 
coastal  plain  of  marine  sediments  is  referred  to;  while  "  marine 
plane  "  indicates  a  wave-cut  plane  surface.  When  we  remember 
that  both  of  these  forms  have  been  called  "  marine  plains," 
the  advantage  of  the  distinction  in  spelling  is  evident.  One  of 
our  ablest  geographers  has  applied  to  a  wave-cut  rock  bench 
along  the  coast  the  term  "  coastal  plain."  Had  he  written 
"  coastal  plane  "  his  meaning  at  least  would  have  been  clear, 
even  though  the  term  as  a  whole  might  still  be  criticized.  A 
peneplain  is  not  "  almost  a  plain  "  of  horizontal  sediments,  but 
is  almost  a  plane  surface  in  the  mathematical  sense  of  the  term ; 

*  "Subaerial"  denudation  was  long  used  for  "fluvial"  denudation,  be- 
cause of  the  prevalent  idea  that  there  were  only  two  important  types  of 
denudation,  subaerial  and  submarine.  Since  the  importance  of  aeolian 
denudation,  which  is  also  subaerial,  has  been  recognized,  it  is  desirable  to 
distinguish  the  two  types  of  subaerial  denudation  by  the  terms  "fluvial" 
and  "aeolian."  It  should  be  understood  that  the  term  fluvial  is  here  used 
in  its  broadest  sense,  to  include  the  action  of  rain  water  assisted  by  weather- 
ing and  all  other  forces  causing  streams  of  water  and  waste  from  the  largest 
to  the  smallest  dimensions.  Fluvial  denudation  is  in  reality  pluvio-fluvial 
denudation. 


PLAINS,   PLANES,   AND   PENEPLANES  167 

therefore,  "  peneplane  "  more  clearly  expresses  the  true  meaning 
of  the  term  than  does  the  older  and  commoner  spelling.  So 
standard  a  text  as  Dana's  "  Manual  of  Geology  "  employs  the 
spelling  "  peneplane  "^\  while  J.  W.  Gregory^^  in  his  recent  book 
on  "  The  Nature  and  Origin  of  Fiords  "  makes  use  of  the  same 
form.  Lawson  has  employed  both  spelUngs,  the  form  "'pene- 
plane "  occurring  in  his  "  Post -Pliocene  Diastrophism  of  the  Coast 
of  Southern  California  "^K  It  may  be,  also,  that  the  combination 
of  "plane"  with  "  pene-"  will  seem  less  objectionable  to  those 
who  dislike  hybrid  terms,  since  "  plane  "  is  closer  to  the  original 
Latin  form  than  "  plain." 

Davis's  introduction  of  "  peneplain  "  into  the  vocabulary  of 
physiography  in  1889"  was  a  valuable  service  to  the  science; 
for  it  led  to  a  speedy  and  world-wide  recognition  of  a  conception 
which  had  previously  been  announced  by  Marvine^^  and  ex- 
tensively developed  by  PowelU*'  and  Dutton^^,  but  which  did  not 
become  current  until  well  named.  It  should  be  appreciated, 
however,  that  at  this  time  the  idea  of  subaerial  denudation  was 
supplanting,  rather  than  supplementing,  the  idea  of  marine 
denudation.  The  general  attitude  was  well  expressed  by  de 
Lapparent^^  in  the  words:  "  La  notion  des  peneplaines  est  ex- 
trement  feconde,  et  ce  n'est  pas  un  de  ses  moindres  merites 
d'avoir  porte  le  coup  de  grace  a  la  theorie  des  plaines  de  denu- 
dation marine,  si  fort  en  honneur  de  I'autre  cote  du  detroit." 
Davis  himself  regarded  extensive  planes  of  marine  denudation 
as  "  very  improbable  "^^  while  planes  of  glacial  or  aeolian  denu- 
dation were  as  yet  scarcely  considered.  All  planes  of  penul- 
timate stages  of  erosion  were  called  "  peneplains,"  because  it 
was  believed  that  they  were  all  formed  essentially  by  subaerial 
agencies.  No  need  was  felt  of  names  for  almost-plane  surfaces 
of  marine  and  other  types  of  erosion,  since  the  existence  of  such 
planes  was  either  considered  improbable,  or  was  not  considered 
at  all. 

Later  years  have  witnessed  the  publication  of  Passarge's 
studies  of  surfaces  of  aeolian  denudation-",  and  the  probability 
of  the  existence  of  fairly  extensive  surfaces  of  marine  denudation 
seems  now  to  be  recognized  by  Davis  and  others-^  We  are 
confronted  with  the  fact  that  there  are  numerous  all-most  plane 
erosion  surfaces  in  various  parts  of  the  world,  the  origin  of  which 
is  in  most  cases  doubtful,  and  in  many  cases  will  probably  never 


168       TERMINOLOGY   AND  CLASSIFICATION   OF   SHORES 

be  known  because  nearly  all  of  the  upland  has  been  destroyed 
by  subsequent  stream  erosion.  As  Davis  once  remarked,  "It 
must  be  remembered  that  the  terms  '  plain  of  marine  denudation  ' 
and  '  peneplain  '  are  in  nearly  all  cases  hardly  more  than  dif- 
ferent names  for  the  same  thing.  If  the  whole  truth  were  known, 
it  is  probable  that  one  or  the  other  name  might  be  appropriately 
applied  in  this  or  that  case,  but  it  is  seldom  that  anyone  has  suc- 
ceeded in  convincing  all  his  contemporaries  that  he  could  dis- 
tinguish a  plain  of  marine  denudation  from  a  peneplain,  or  vice 
versa  "-.  Present  needs  can  better  be  met  by  applying  the 
excellent  term  "  peneplane  "  to  all  these  surfaces,  and  qualifying 
the  term  by  the  word  fluvial,  marine,  aeolian  or  glacial  in  case 
it  can  be  shown  that  a  given  surface  is  of  the  origin  indicated  by 
the  qualifier,  rather  than  by  inventing  a  new  term  for  almost- 
plane  erosion  surfaces  of  doubtful  origin.  Peneplane  is  too  valu- 
able a  term,  and  is  too  extensively  used,  to  have  it  restricted  to  the 
very  few  (if  any)  erosion  surfaces  demonstrably  of  fluvial  origin; 
and  since  the  wise  physiographer  must  avoid  a  name  which 
commits  him  unwillingly  to  a  certain  theory  of  origin,  it  is  best  in 
the  present  case  to  extend  the  meaning  of  the  term. 

While  it  is  true  that  I  am  advocating  a  broader  significance  for 
"  peneplane  "  than  is  usually  given  to  it,  precedent  for  such 
usage  is  not  altogether  lacking.  H.  E.  Gregory^s  is  respon- 
sible for  using  "  peneplain  "  as  synonymous  with  "  plain  of 
denudation  .  .  .  carved  out  of  other  land  forms  either  by  the 
action  of  the  forces  that  work  on  the  land  or  by  the  waves  of  the 
sea."  Davis  himself  has  occasionally  employed  the  term  in  the 
broader  sense.  Thus,  in  an  account  of  the  geographic  develop- 
ment of  Northern  New  Jersey,  he  discusses  at  length  "  whether 
the  old  Highland  peneplain  was  the  product  of  subaerial  or  sub- 
marine processes  "24.  He  distinguishes  between  "subaerial  base- 
level  plain"  and  "submarine  platform,"  but  applies  the  name 
"peneplain"  to  the  topographic  feature  itself  while  its  origin 
remains  in  doubt.  In  a  discussion  of  the  arid  cycle  by  the 
same  author-^  "  peneplains  "  are  usually  contrasted  with  true 
"  plains  "  of  aeolian  erosion;  but  the  frequent  use  of  the  ex- 
pression "  normal  peneplain,"  and  the  application  of  the  term 
"  monadnock  "  to  residuals  on  a  plane  of  aeolian  denudation, 
suggest  that  the  author  at  least  unconsciously  recognized  the 
possible  existence  of  "  peneplains  "  which  were  not  formed  by 


CLASSIFICATION   OF  SHORELINES  169 

"  normal  "  (stream)  erosion.  Marine  peneplanes  are  more  defi- 
nitely recognized,  at  least  as  a  theoretical  possibility  requiring 
discussion,  when  in  an  essay  on  planes  of  marine  and  subaerial 
denudation  the  author  says:  "  By  whatever  process  the  so-called 
'  plain  of  denudation  '  was  produced,  an  explanation  that  will 
account  for  a  peneplain  of  moderate  or  slight  relief  is  all  that 
is  necessary  "2^. 

Recognition  of  the  fact  that  wave  erosion  is  capable  of  produc- 
ing a  marine  plane,  or  at  least  a  marine  peneplane,  is  essential  to  a 
full  comprehension  of  some  significant  phases  of  shoreline  activity. 
This  matter  will  claim  our  attention  in  the  next  chapter. 

Classification  of  Shorelines.  —  Various  classifications  of  shore- 
lines or  coasts  have  been  proposed,  some  of  which  are  based  on 
form  rather  than  genesis,  while  others  take  account  of  the  origin 
of  shorelines  but  do  not  consider  the  stages  of  development 
reached  since  they  originated.  The  first  type  of  classification  is 
wholly  empirical  and  therefore  not  very  significant;  the  second 
type  is  partly  genetic,  but  not  evolutionary,  and  is  therefore  less 
significant  than  it  might  be.  Neither  type  permits  one  to  ar- 
range all  shore  forms  in  genetic  series  according  to  their  relative 
advance  in  the  cycle  of  shoreline  evolution.  Good  examples 
of  such  classifications  will  be  found  in  Suess'  "  Das  Antlitz  der 
Erde  "^^  von  Richthofen's  "  Fiihrer  fur  Forschungsreisende  "-^ 
and  Penck's  "  Morphologic  der  Erdoberflache  "^a.  Those  desiring 
to  study  further  the  methods  of  classification  here  referred  to  will 
profit  from  a  reading  of  Fischer's  paper  entitled  "Zur  entwickel- 
ungs-geschichte  der  Kiisten  "^^^  and  Hahn's  paper  on  "  Kii- 
steneinteilung  und  Kustenentwickelung  im  verkehrsgeographis- 
chen  Sinne  "^^  Applications  of  such  methods  in  the  description 
of  specific  coasts  have  been  made  by  Haage^-  in  his  dissertation 
on  "  Die  Deutsche  Nordseekiiste,"  by  MeinhokP''  in  an  essay 
entitled  "  Die  Kuste  der  mittleren  atlantischen  Staaten  Nor- 
damerikas,"  and  by  Weidemiiller^^  in  his  account  of  "  Die 
Schwemmlandkiisten  der  Vereinigten  Staaten  von  Nordamerika." 
American  students  will  be  especially  interested  in  the  last  two 
papers,  since  they  relate  to  our  own  shores  and  at  the  same  time 
furnish  good  examples  of  a  type  of  physiographic  description 
very  commonly  encountered  in  German  writings.  The  details 
of  coastal  features  are  empirically  described  with  painstaking 
care  and  at  great  length,  instead  of  being  represented  by  maps; 


170       TERMINOLOGY   AND   CLASSIFICATION   OF   SHORES 

and  while  the  origin  of  the  features  so  described  is  then  con- 
sidered, each  coast  section  is  for  the  most  part  treated  as  a 
special  isolated  problem,  without  regard  to  its  position  in  a 
series  of  sequential  forms. 

Numerical  Methods.  —  Many  attempts  have  been  made  to  ex- 
press the  distinctions  between  different  types  of  coasts  in  numeri- 
cal terms.  This  method  has  been  much  in  vogue  among  German 
students  since  the  time  of  Ritter.  The  essential  object  of  the 
method  is  to  establish  a  comparison  between  coasts  showing  differ- 
ent degrees  of  indentation  by  the  sea,  and  the  comparison  is  usually 
expressed  by  a  numerical  relation  rather  than  by  absolute  figures, 
such  as  the  actual  length  of  shoreline.  Various  relations  have 
been  utilized  in  this  connection,  such  as  the  ratio  existing  between 
the  length  of  the  actual  shoreline  and  the  shortest  possible  shoreline 
which  the  area  in  question  could  have  (Nagel);  or  the  ratio  of 
actual  shoreHne  length  to  the  length  of  an  ideal  contour  con- 
necting the  outer  points  of  the  peninsulas,  or  the  innermost 
points  of  the  bayheads;  or  the  ratio  between  the  area  of  the 
main  continental  mass  and  the  area  of  the  outlying  peninsulas 
and  islands  (Kloden) .  Ritter^^  and  Berghaus^'^  compared  the  area 
of  the  land  with  the  length  of  the  bordering  shore,  a  method  which 
was  criticized  by  mathematicians  on  the  ground  that  planes  could 
not  properly  be  compared  with  lines.  NageP^  determined  the 
size  of  a  circle  containing  the  same  area  as  the  land  whose  coast 
was  under  examination,  and  then  compared  the  length  of  actual 
shoreline  with  the  circumference  of  this  circle. 

Schwind^^  employs  .a  comparison  between  the  length  of  actual 
shoreline  and  the  length  of  a  selected  isobath  having  a  much 
simpler  form,  thus  following  the  method  of  his  teacher,  RatzeF, 
who  considers  that  the  length  of  the  shoreline  should  always 
be  compared  with  some  real  contour-line.  De  Martonne"*" 
presents  a  number  of  valid  criticisms  against  all  these  methods, 
and  suggests  that  more  significant  results  can  be  obtained  by 
comparing  lengths  of  shoreline  with  areas  included  between 
selected  contours  above  and  below  sealevel. 

These  are  but  a  few  of  the  great  number  of  schemes  devised  by 
different  students  in  an  attempt  to  discover  a  method  which 
would  not  be  open  to  the  criticisms  urged  by  each  student  against 
all  the  methods  of  his  predecessors.  We  must  restate  today 
Hahn's  conclusion*^  of  a  third  of  a  century  ago,  that  all  numerical 


CLASSIFICATION  OF  SHORELINES  171 

methods  of  coast  description  are  failures.  All  of  them  are  es- 
sentially empirical,  and  hence  of  little  or  no  significance  to  the 
student  of  shore  forms.  They  tell  little  which  a  good  map  does 
not  tell  much  better.  Even  when  the  numerical  expression  is 
combined  with  a  discussion  of  the  relation  of  the  shoreline  to 
geological  structure,  changes  of  level,  the  progress  of  shore 
accretion,  and  other  phenomena  affecting  the  coast,  the  result 
is  a  description  only  partially  genetic,  and  one  which  fails  to 
recognize  the  importance  of  shore  processes  in  developing  the 
shoreline  in  orderly,  sequential  stages. 

The  reader  who  would  examine  further  the  numerical  schemes 
for  coastal  description  will  find  a  good  historical  review  of  the 
development  of  the  method  in  Riessen's  "  Uberblick  und  Kritik 
der  Versuche  Zahlenausdriicke  fiir  die  grossere  oder  geringere 
Klistenentwickelung  eines  Landes  oder  Kontinentes  zu  finden"'*'- 
while  Schwind's  essay  on  "  Die  Riasktisten  und  ihr  Verhaltnis 
zu  den  Fjordkiisten  unter  besonderer  Beriicksichtigung  der 
horizontalen  Gliederung,"'^^  contains  a  bibliography  of  the  sub- 
ject. Reference  should  also  be  made  to  the  paper  by  de  Mar- 
tonne^  already  mentioned  and  to  other  papers  by  Reuschle*^, 
Schroter*^,  and  HentzscheP^  where  special  applications  of  the 
method  are  considered.  A  good  idea  of  the  ingenious  but  highly 
artificial  and  complex  mathematical  methods  of  describing  coastal 
embayments  employed  by  Weule,  Giittner,  and  others  may  be 
secured  from  Giittner's  dissertation  on  "  Geographische  Ho- 
mologien  an  den  Kiisten  mit  besonderer  Beriicksichtigung  der 
Schwemmlandkiisten  "'^^.  Descriptions  of  the  southeastern  coast 
of  the  United  States  based  in  part  on  numerical  methods  will  be 
found  in  reports  by  Weule^^  and  Weidemiiller^". 

Genetic  Classification  of  Shorelines.  — ■  The  character  of  any  shore- 
line must  depend  in  the  first  instance  upon  the  character  of  the 
land  surface  against  which  the  sea  comes  to  rest.  If  the  surface 
is  a  partially  submerged,  irregularly  dissected  land  area  with 
numerous  hills  and  valleys,  the  water  will  enter  the  branching 
valleys  and  spread  around  the  hills,  forming  a  very  complicated 
shoreline.  If  the  surface  is  a  smooth,  emerged  sea-bottom,  the 
shoreline  is  necessarily  simple.  It  follows  from  this  that  the 
most  significant  classification  of  shorelines  will  be  one  which  takes 
account  of  the  nature  of  the  movements  of  land  or  water  which 
brought  the  water  surface  against  the  land  at  the  present  level. 


172       TERMINOLOGY  AND  CLASSIFICATION  OF   SHORES 

It  was  upon  such  a  genetic  basis  that  Davis^^  divided  shoreHnes 
into  two  primary  classes,  and  that  the  more  detailed  discussion 
of  Gulliver^2  ^y^s  founded.  There  are,  however,  important  shore- 
lines which  find  no  satisfactory  place  in  the  classifications  of  Davis 
and  Gulliver,  and  for  which  provision  must  be  made. 

We  will  find  it  profitable  to  divide  shorelines  into  four  main 
classes:  I,  Shorelines  of  submergence,  or  those  shorelines  pro- 
duced when  the  water  surface  comes  to  rest  against  a  partially 
submerged  land  area;  II,  Shorelines  of  emergence,  or  those  result- 
ing when  the  water  surface  comes  to  rest  against  a  partially 
emerged  sea  or  lake  floor;  III,  Neutral  Shorelines,  or  those  whose 
essential  features  do  not  depend  on  either  the  submergence  of 
a  former  land  surface  or  the  emergence  of  a  former  subaqueous 
surface;  IV,  Compound  Shorelines,  or  those  whose  essential 
features  combine  elements  of  at  least  two  of  the  preceding  classes. 

Shorelines  of  submergence  have  been  called  "  shorelines  of 
depression";  but  this  implies  a  depression  of  the  land,  whereas 
the  submergence  may  as  well  result  from  a  rising  of  sea  or  lake 
level,  or  from  the  melting  of  tidewater  glaciers  permitting  sub- 
mergence of  glacial  troughs  without  any  change  in  the  relative 
level  of  either  land  or  water.  The  term  "  shorelines  of  emergence  " 
is- likewise  preferable  to  "  shorelines  of  elevation,"  not  only  be- 
cause emergence  may  result  from  the  lowering  of  sea  or  lake 
surface,  but  for  the  added  reason  that  "  shorelines  of  elevation  " 
and  "  elevated  shorelines,"  two  wholl}^  distinct  forms,  are  in 
danger  of  having  their  similar  titles  confused  even  though  there 
is  no  possibility  of  confusing  the  forms  themselves.  The  terms 
"  sinking"  and  "  rising"  have  been  applied  to  coasts  bordered 
by  shorelines  of  submergence  and  shorelines  of  emergence;  and 
in  his  admirable  treatise  on  "  Die  Erklarende  Beschreibung  der 
Landformen  "  Davis^-^  classifies  the  coasts,  rather  than  the  shore- 
lines, into  Senkungskiisten  and  Hebungskiisten.  Such  terms  are 
open  to  the  objection  that  they  not  onl}^  imply  an  actual  change 
of  level,  and  that  it  is  a  land  movement  which  effects  this  change, 
but  also  that  the  movement  is  still  going  on;  three  implications 
which  are  probably  not  justified  in  the  case  of  many  coasts  to 
which  the  terms  are  applied.  We  might  employ  the  terms 
"  positive  shorelines  "  and  "  negative  shorelines,"  thus  indicating 
that  the  shorelines  resulted  from  positive  or  negative  movements 
of  the  water  line,  without  indicating  whether  such  movements 


SHORELINES  OF  SUBMERGENCE  173 

resulted  from  changes  in  the  level  of  the  land  or  the  water.  But 
these  terms  are  open  to  two  objections:  they  have  been  applied 
to  land  movements  as  well  as  to  strand  movements,  and  many 
are  confused  by  the  necessity  of  remembering  that  a  positive 
land  movement  means  a  negative  strand  movement,  whereas 
a  positive  water  movement  means  a  positive  strand  movement; 
furthermore,  they  imply  that  some  vertical  change  in  either  land 
or  water  is  necessary,  whereas  we  have  seen  that  submergence 
may  occur  with  no  change  in  the  level  of  either.  "  Irregular 
shorelines  "  and  "  simple  shorelines  "  are  unsatisfactory,  both 
because  they  are  empirical  terms  which  carry  no  suggestion  of 
origin,  and  because  all  classes  of  shorelines  are  simple  in  mature 
or  late  mature  stages  of  development.  "  Shorelines  of  sub- 
mergence "  and  "  shorelines  of  emergence  "  are  explanatory 
terms;  they  are  genetic  rather  than  empirical;  they  do  not  carry 
any  implication  as  to  whether  it  is  the  land  or  the  sea  which 
moves,  and  do  not  even  imply  any  vertical  change  of  level  in 
either;  they  are  easily  understood,  and  are  not  in  danger  of  being 
confused  with  other  terms  applied  to  shoreline  phenomena.  For 
these  reasons  it  seems  desirable  to  use  them  instead  of  the  other 
terms  which  have  been  mentioned. 

I.  Shorelines  of  submergence  may  be  sulxlivided  into  two 
main  types,  according  to  the  nature  of  the  forms  submerged. 
These  are:  (a)  shorelines  formed  l)y  the  partial  submergence  of  a 
land  mass  dissected  by  normal  river  valleys,  which  may  be  called 
ria  shorelines,  after  the  ria  coast  of  northwestern  Spain,  which  was 
produced  by  the  drowning  of  normal  river  valleys  along  a  moun- 
tainous coast;  thus  used,  the  term  ria  is  not  restricted  to  the 
narrow  meaning  assigned  to  it  by  von  Richthofen,  who  first  used 
it  in  a  generic  sense;  but  is  employed  in  the  broader  sense  in 
which  it  has  been  used  by  Gulliver  and  others :  (5)  shorelines  pro- 
duced by  the  partial  submergence  of  a  region  of  glacial  troughs, 
known  sls  fjord  shorelines,  like  those  of  Norway  and  Alaska. 

(a)  Ria  Shorelines.  Since  ria  shorelines  are  produced  by  the 
partial  submergence  of  normally  dissected  examples  of  the  three 
main  groups  of  constructional  landforms  (plains-plateaus,  moun- 
tains and  volcanoes),  we  may  recognize  as  three  subtypes: 
embayed  plain  or  plateau  shorelines,  such  as  are  found  in  the 
Chesapeake  Bay  region  (Fig.  24) ;  embayed  mountain  shorelines,  of 
which  the  Maine  coast  and  the  coast  of  Brittany  furnish  good  ex- 


174     TERMINOLOGY   AND   CLASSIFICATION  OF  SHORES 


SHORELINES  OF   SUBMERGENCE  175 

Plate  XVI. 


Homviken,  a  small  fjord  near  North  Cape,  Norway,  showing  typical  over- 
steepened  walls  of  a  glacial  trough. 


176       TERMINOLOGY  AND   CLASSIFICATION  OF   SHORES 

amples;  and  embayed  volcano  shorelines,  a  number  of  which  are 
found  in  the  South  Pacific,  having  been  cited  by  Dana^  in  support 
of  Darwin's  theory  of  a  subsidence  of  that  ocean's  floor.  When 
desirable,  the  form  of  the  shoreline  may  be  more  clearly  in- 
dicated by  specifying  the  particular  type  of  plain,  plateau, 
mountain,  or  volcano  involved.  Thus  one  may  speak  of  the 
embayed  coastal  plain  shoreline  of  Maryland,  or  the  embayed 
folded  mountain  shoreline  of  the  Adriatic  coast  of  Dalmatia, 
thereby  taking  due  account  of  the  structure  of  the  thing  sub- 
merged, which  is  an  important  element  in  determining  the 
character  of  any  shoreline  of  submergence. 

Care  must  be  taken  to  avoid  the  ancient  fallacy  that  the 
branching  bays  of  ria  coasts  are  due  to  wave  and  tidal  erosion^^. 
It  is  illustrative  of  the  slow  progress  of  the  science  of  shorelines 
that  Playfair,  at  the  same  time  that  he  made  his  keen  obser- 
vations on  the  nature  and  origin  of  river  valleys,  should  ascribe 
deep  gulfs  and  salient  promontories  to  differential  wave  erosion,^^ 
and  that  nearly  a  century  later  Fischer^^  and  other  students  of 
shorelines  should  be  found  still  advocating  this  view.  Al- 
though it  was  recognized  some  time  ago  that  as  a  rule  tidal  cur- 
rents merely  ebb  and  flow  through  submerged  branching  river 
valleys  which  they  had  no  power  to  originate  and  which  or- 
dinarily they  have  but  very  moderate  power  to  enlarge,  and  that 
wave  erosion  tends  to  obliterate  the  larger  irregularities  of  a 
coast  and  not  to  make  them,  one  still  finds  the  tidal  and  wave 
origin  of  such  drowned  valleys  as  those  of  the  Maine  coast 
maintained  in  recent  editions  of  a  standard  textbook  on  geology^^. 

(6)  Fjord  Shorelines.  —  Perhaps  no  type  of  shoreline  has  given 
rise  to  so  much  discussion  as  has  the  fjord  shoreline.  We  may 
note  in  the  first  place  that  geologists  and  geographers  may  be 
divided  into  two  main  groups  whose  ideas  regarding  the  origin 
of  fjords  are  mutually  opposed.  The  first  group  may  be  desig- 
nated as  the  "  glacialists,  "  because  in  their  opinion  all  the 
phenomena  peculiar  to  fjords  may  be  explained  as  the  result -of 
extensive  glacial  over-deepening  of  pre-glacial  river  valleys  near 
the  sea.  The  second  group,  or  ''  non-glacialists,"  reject  the 
theory  of  ice  erosion,  and  attempt  to  account  for  the  phenomena 
of  fjords  in  other  ways. 

According  to  the  glacial  theory,  fjords  are  partially  submerged 
glacial  troughs.     The  troughs  of  glaciated  mountains  far  from 


Fig.  25.  —  A  typical  section  of  the  fjord  coast  of  Norway,  showing  angular 

pattern  attributed  to  fault-control.  (177) 


178     TERMINOLOGY   AND   CLASSIFICATION   OF   SHORES 

Plate  XVII. 


Photo  by  Underwoofl  &  Underwood. 

Idde  Fjord  near  Fredriksten,  Norway,  showing  rectangular  pattern  char- 
acteristic of  many  fjord  coasts. 


SHORELINES  OF  SUBMERGENCE  179 

the  sea  are  similar  to  fjords,  except  that  the  former  have  not 
been  drowned  by  marine  waters.  In  both  cases  the  troughs  were 
formed  by  extensive  glacial  over-deepening  of  former  river  val- 
leys. The  pre-glacial  valleys  guided  the  glaciers  which  later 
came  to  occupy  them,  and  by  confining  the  ice  streams  to  the 
narrow  limits  imposed  by  the  valley  walls,  insured  a  maximum 
efficiency  of  glacial  erosion.  The  glacial  theory  asks  no  questions 
as  to  what  determined  the  courses  of  the  pre-glacial  valleys; 
but  it  is  fully  recognized  that  among  other  causes  ancient  fault 
lines  must  be  considered,  since  a  fault  may  give  a  crushed  zone 
which  is  weaker  than  the  unfractured  rock,  or  may  bring  a  belt 
of  weak  rock  into  such  position  that  subsequent  valleys  will 
soon  be  excavated  along  it,  parallel  to  the  fault.  This  would 
satisfactorily  account  for  the  fact  that  many  fjord  shorelines  have 
a  more  or  less  angular  pattern.     (Fig.  25  and  Plate  XVII). 

Esmark^^  was  the  first  to  advocate  the  glacial  origin  of  fjords, 
almost  a  century  ago.  The  fjord  valleys  of  New  Zealand  were 
ascribed  largely  to  ice  erosion  by  von  Haast^"  in  1865,  while 
Helland^^  a  few  years  later,  in  discussing  the  fjords  of  Norway 
and  Greenland,  gave  the  best  exposition  of  the  glacial  theory  as 
applied  to  the  interpretation  of  fjords  which  had  appeared  up 
to  that  time.  Helland  seems  to  have  anticipated  Shaler  in 
recognizing  the  ability  of  glaciers  to  excavate  their  channels 
below  sealevel,  and  to  have  given  a  fairly  good  account  of  the 
essential  significance  of  hanging  valleys  some  twenty  years  be- 
fore Gannett's  classic  statement.  The  influence  of  rock  frac- 
tures on  the  orientation  of  fjord  valleys  was  recognized  by 
Brogger^^^  -^ho  did  not  fail,  however,  to  attribute  the  actual 
excavation  of  the  fjords  to  glacial  erosion.  In  a  similar  manner 
Reusch^^  for  the  Norwegian  fjords  and  Andrews^  for  those  of 
New  Zealand,  make  a  clear  distinction  between  the  role  of  faulting 
in  determining  lines  of  weakness  favorable  to  rapid  stream  and 
glacial  erosion,  and  the  role  of  glaciers  in  giving  to  the  fjords 
their  present  form  and  depth. 

In  1895  Shaler^^,  in  discussing  changes  of  sealevel,  accepted 
the  glacial  origin  of  fjords  and  stated  that  since  glaciers  may  cut 
their  channels  below  the  surface  of  the  sea,  the  flooding  of  a 
glacial  trough  may  be  accomplished  as  the  ice  melts,  without  any 
sinking  of  the  land  or  rising  of  the  water  level.  This  same  view, 
that  fjords  do  not  indicate  past  changes  of  level,  was  adopted 


180       TERMINOLOGY   AND   CLASSIFICATION  OF  SHORES 


Plate  XVIII. 


P)ujt,j  by  Uiuhrii.mil  &  Underwood. 

The  Naero  Fjord,  Norway,  a  partially  submerged  glacial  trough. 


SHORELINES  OF  SUBMERGENCE  181 

by  Hubbard"^  in  a  brief  review  of  the  fjord  problem  which  he 
published  in  1901;  by  Daly«^  in  his  account  of  the  Labrador 
fjords;  and  by  Andrews*'^  in  discussing  the  fjords  of  New  Zealand. 
It  is  further  elaborated  by  Gilbert''^  in  his  report  on  glacial  studies, 
forming  the  third  volume  of  the  Harriman  Alaska  Series,  where 
the  reader  will  find  a  discussion  of  the  physics  of  glacial  erosion 
below  sealevel.  MarshalP"  in  his  "  Geography  of  New  Zealand," 
and  Tarr^i  in  his  report  on  the  "  Physiography  and  Glacial 
Geology  of  the  Yakutat  Bay  Region,  Alaska  "  are  among  other 
students  of  fjords  who  attribute  their  excavation  to  ice  erosion. 

Members  of  the  non-glacialist  group  are  by  no  means  in  agree- 
ment among  themselves  as  to  the  origin  of  fjords.  They  agree 
on  one  thing  only  —  that  ice  did  not  excavate  these  deeply  sub- 
merged canyons.  Some  consider  fjords  the  product  of  normal 
stream  erosion  followed  by  a  partial  submergence  which  per- 
mitted the  valleys  to  be  drowned.  This  was  the  view  expressed 
by  Dana'^^  who  first  emphasized  the  restriction  of  fjords  to  high 
latitudes  but  did  not  suggest  for  them  a  glacial  origin.  Upham^^ 
definitely  rejects  the  glacial  explanation,  and  follows  Dana  in 
considering  fjords  as  drowned  normal  river  valleys.  Brigham^'* 
and  HulP^  seem  to  incline  to  the  same  view,  the  former  speaking 
of  "  the  common  sense  conclusion  that  they  are  river  valleys 
made  tidal  by  drowning";  but  both  recognize  that  fjords  have 
been  to  some  extent  modified  iDy  glaciers.  Hirt^'''  in  a  review  of 
"  Das  Fjord-Problem,"  Dinse^^  in  a  more  elaborate  study  of 
"  Die  Fjordbildungen,"  and  Grossman  and  Lomas'^^  in  a  report 
on  the  Faroe  Islands  tend  to  assign  to  glaciers  but  a  moderate 
role  in  modifying  pre-existing  valleys;  while  J.  W.  Tayler^^  and 
Fairchild^"  definitely  reject  the  glacial  theory  of  fjord  formation, 
Fairchild  specifically  invoking  coastal  subsidence  to  account 
for  the  fjord  embayments. 

Among  those  students  who  admit  that  ice  erosion  played  an 
essential  part  in  fashioning  fjord  valleys,  there  are  a  number  who 
either  expressly  require  coastal  subsidence,  or  else  tacitly  assume 
that  subsidence  is  necessary  for  the  drowning  of  the  glacial 
troughs.  Robert  Brown'^^  writing  on  the  "  Formation  of  Fjords  " 
in  1869  and  1871,  required  the  combined  action  of  glacial  erosion 
and  coastal  subsidence.  The  same  view  is  supported  by  Rem- 
mers*2  in  his  ''  Untersuchungen  der  Fjorde  an  der  Kiiste  von 
Maine,"    and   by    Giittner^   in    an   essay   on    "  Geographische 


182       TERMINOLOGY   AND   CLASSIFICATION  OF  SHORES 

Homologien  an  den  Kiisten  "  published  in  1895.  Those  writers 
assuming  the  necessity  of  subsidence  without  specifically  dis- 
cussing the  point,  include  Penck^  in  his  "  Morphologic  der 
Erdoberflache,"  de  Lapparent^^  in  his  "  Traite  de  Geologie," 
Gallois^^  in  his  account  of  "  Les  Andes  de  Patagonie,"  Le  Conte^'^ 
in  his  "  Elements  of  Geology,"  and  Hobbs^^  in  his  "  Earth 
Features  and  Their  Meaning." 

Formerly  many  observers  were  inclined  to  regard  every  fjord 
as  either  a  rift  valley  formed  by  the  dropping  down  of  a  narrow 
strip  of  the  earth's  crust  between  two  parallel  faults,  or  as  a 
gaping  chasm  opened  along  a  single  fault.  This  tectonic  theory 
of  the  origin  of  fjords,  once  much  in  vogue  as  an  explanation  for 
all  valleys,  is  now  generally  regarded  as  obsolete.  Statements  of 
the  tectonic  theory  in  which  ice  is  credited  with  a  very  minor 
role  in  clearing  out  crushed  and  broken  rock  left  in  the  fault 
cleft,  or  in  the  moderate  widening  of  an  open  chasm,  will  be 
found  in  a  short  paper  by  Gurlt^^,  entitled  "  Uber  die  Entste- 
hungsweise  der  Fjorde,"  published  in  1874,  in  Peschel's  "  Neue 
Probleme  der  vergleichenden  Erdkunde  als  Versuch  einer 
Morphologie  der  Erdoberflache  "^^,  dated  four  years  later;  and 
in  Kornerup's  account  of  the  fjords  of  southwest  Greenland^^ 
A  more  modern  supporter  of  the  tectonic  origin  of  fjords  is 
Steffen^-  in  a  short  paper  on  "  Der  Baker-Fjord  in  West- 
Patagonien."  But  by  far  the  most  elaborate  thesis  in  support 
of  the  tectonic  theory  is  J.  W.  Gregory's  recent  book  on  "  The 
Nature  and  Origin  of  Fjords  "^^.  This  serious  attempt  to  re- 
habilitate a  much-discredited  theory  of  fjord  origin  contains 
extensive  references  to  the  literature  of  fjords,  but  frequently 
misinterprets  the  views  held  by  the  authors  quoted.  In  a  critical 
review  of  the  book  the  present  writer^**  has  endeavored  to  show 
that  Gregory's  arguments  are  based  upon  a  misconception  of 
what  the  glacial  theory  of  fjords  implies,  and  upon  an  uncertain 
and  variable  interpretation  of  the  tectonic  theory. 

Readers  who  wish  to  follow  the  discussion  of  the  fjord  prob- 
lem further  will  be  interested  in  an  essay  by  Nordenskjold^^ 
on  "  Topographisch-geologische  Studien  in  Fjordgebieten  "  and 
in  a  shorter  paper  by  Werth^^  entitled  "  Fjorde,  Fjarde,  und 
Fohrden."  Both  contain  many  references  to  the  literature  of 
the  subject,  and  Werth's  paper  explains  the  differences  between 
typical  fjords,  the  allied  forms  in  low  rocky  coasts  like  south- 


SHORELINES   OF  SUBMERGENCE 


183 


Plate  XIX. 


Photo  blj  Ln.l.nv, .<...!  a     I    :    i.nr.,..!. 

Lake  Loen,  Norway,  occupying  a  glacial  trough  and  practically  continuous 
with  the  upper  part  of  Nord  Fjord.  Compare  with  similar  topography 
shown  on  Plate  XVIII. 


184     TERMINOLOGY  AND   CLASSIFICATION  OF  SHORES 

western  Sweden  sometimes  called  fiards  (Plate  XX),  and  the 
Johrden  of  the  Baltic  shores  of  Denmark  and  Schleswig-Hol- 
stein,  similar  to  fiards  but  lacking  their  rocky  shores.  The 
relations  of  these  three  sub-types  of  fjords  are  also  considered 
by  Penck97,  Dinse9^  and  Hubbard^^  An  early  paper  by  RatzeP^^ 
discusses  at  some  length  the  essential  characteristics  of  fjords. 

Without,  at  this  time,  entering  into  any  elaborate  discussion 
of  the  several  theories  of  fjord  formation,  it  may  be  said  that  the 
interpretation  which  would  regard  fjords  as  partiall}'  submerged 
river  valleys  fails  to  account  on  any  rational  basis  for  the  re- 
striction of  true  fjords  to  glaciated  high  latitudes,  for  the 
identity  in  form  between  fjord-valleys  andihe  glacial  troughs 
of  glaciated  high  altitudes,  for  the  almost  uniform  violation  of 
Playfair's  law  by  tributary  valleys  which  enter  main  fjords  with 
discordant  junctions,  and  for  the  occurrence  of  submerged  fjord 
basins  which,  were  the  land  to  stand  higher,  would  become  lake 
basins  not  distinguishaljle  from  those  of  typical  glacial  troughs. 
Special  pleading  and  strained  reasoning  have  suggested  a  variety 
of  possible  explanations  for  each  of  these  characteristic  relation- 
ships, some  of  which  might  apply  in  one  given  instance,  others 
in  another.  Glacial  over-deepening  of  pre-existing  river  valleys 
alone  offers  a  single  explanation  adequate  to  account  at  once  for 
all  of  the  specified  relationships  in  all  of  the  observed  cases. 

The  tectonic  theory  of  fjords  is  based  on  a  misunderstanding 
of  the  significance  of  the  known  occurrence  of  fault-lines  in 
certain  fjords,  and  of  the  rectangular  pattern  of  other  fjords, 
which  suggests  an  intersecting  fault  pattern.  There  can  be 
little  doubt  but  that  crushed  zones  along  faults,  and  infaulted 
strips  of  weak  rook,  have  often  determined  the  position  and 
pattern  of  fjord-valleys.  It  is,  however,  an  error  of  reasoning 
to  jump  to  the  conclusion  that  faults  make  fjords.  As  already 
noted,  the  glacial  theory  of  fjord  origin  fully  recognizes  the  fact 
that  the  pre-glacial  valleys  later  transformed  into  fjords  were 
often  excavated  along  ancient  fault-lines.  Stream  erosion  natu- 
ally  took  advantage  of  the  weak  belts  determined  by  faulting, 
forming  fault-line  valleys;  but  not  until  ice  occupied  these  pre- 
glacial  stream  valleys  and  profoundly  changed  their  shape  and 
their  depth,  were  the  forms  which  we  called  fjords  produced. 
To  prove  the  presence  of  a  fault-line  through  a  fjord  is,  therefore, 
to  prove  nothing  as  to  the  tectonic  origin  of  the  fjord.     The 


SHORELINES  OF  SUBMERGENCE 


185 


186       TERMINOLOGY   AND   CLASSIFICATION  OF  SHORES 

tectonic  theory,  moreover,  affords  no  rational  explanation  of  the 
restriction  of  fjords  to  high  latitudes,  nor  of  the  identity  in  form 
between  fjord  valleys  and  unsubmerged  glacial  troughs,  between 
fjord-basins  and  trough  lake-basins.  In  the  glacial  theory  alone 
do  all  of  the  phenomena  cited,  including  the  relation  of  fjords  to 
faults,  find  a  logical  interpretation. 

It  should  be  noted  that  the  subsidence  of  the  land,  which  was 
an  essential  element  of  the  theory  that  fjords  are  drowned  normal 
river  valleys,  ha^s  been  specifically  invoked  or  tacitly  assumed  by 
many  of  the  supporters  of  the  tectonic  and  glacial  theories. 
Whatever  may  be  said  regarding  the  discarded  tectonic  theory, 
it  is  clearly  an  error  of  reasoning  which  would  assume  the  neces- 
sity of  land  sinking  in  order  to  account  for  partial  submergence 
of  glacially  over-deepened  valleys.  Glaciers  in  high  latitudes 
reach  the  sea  at  the  present  time;  and  glaciers  cannot  cease  to 
erode  their  channels  until  the  ice  is  floated,  which  in  turn  cannot 
occur  until  the  glacier  has  cut  something  like  six-sevenths  of  its 
thickness  below  sealevel.  Shaler  was  clearly  right  in  stating 
that  the  over-deepened  channel  of  such  a  glacier  would  be  flooded 
by  the  sea  as  the  ice  melted,  without  any  sinking  of  the  land.  It 
is  important  to  remember,  therefore,  that  there  is  no  solid 
ground  for  the  popular  opinion  that  fjords  are  an  indication  of 
land  subsidence. 

II.  Shorelines  of  Emergence.  —  The  typical  shoreline  of 
emergence  is  the  coastal  plain  shoreline,  resulting  from  the  emer- 
gence of  a  submarine  or  sublacustrine  plain.  Whatever  may 
have  been  the  initial  inequalities  of  a  given  sea-bottom  or  lake- 
bottom,  deposition  of  sediment  will  in  time  obliterate  them.  The 
resultant  smooth  bottom  is  protected  from  the  action  of  those 
subaerial  agents  of  erosion  which  normally  give  to  the  lands 
their  remarkable  variety  of  relief.  Waves  and  currents  tend 
to  reduce  irregularities  of  the  subaqueous  surface,  not  to  produce 
them.  We  should,  therefore,  expect  that  most  shorelines  of 
emergence  would  be  coastal  plain  shorelines;  and  this  indeed 
seems  to  be  the  case. 

It  is  conceivable  that  an  irregular,  dissected  land  mass  might 
first  be  submerged,  and  then  experience  partial  emergence  before 
there  was  time  for  subaqueous  processes  to  obliterate  the  ir- 
regularities. In  such  a  case  the  shoreline  would  be  classed  as 
a  shoreline  of  submergence,  since  all  its  chief  characteristics  are 


NEUTRAL  SHORELINES 


187 


determined  by  the  earlier  major  movement  of  submergence  and 
not  by  the  later  minor  emergence.  This  is  the  case  with  the 
extremely  irregular  shoreline  of  Maine,  which  is  frequently  cited 
as  a  type  example  of  the  shoreline  of  submergence,  notwithstand- 
ing a  late  uplift  of  the  coast  of  moderate  amount.  Similarly  the 
coastal  plain  of  southern  New  Jersey  and  the  coastal  plain  of 
Texas  afford  good  examples  of  shorelines  of  emergence,  although 
a  later  slight  submergence  has  resulted  in  moderate  embayment 
of  the  inner  shorelines  behind  the  offshore  bars. 

On  theoretical  grounds  one  might  discuss  other  types  of  shore- 
lines of  emergence,  as,  for  example,  the  shoreline  which  would 
be  formed  if  an  original  submarine  volcano  were  raised  partially 
above  sealevel  by  the  upwarping  of  the  ocean  floor.  Such  dis- 
cussion would  not,  however,  materially  add  to  our  understanding 
of  the  principles  of 
shoreline  develop- 
ment, and  may  better 
be  left  to  those  who 
in  the  future  encoun- 
ter examples  of  such 
shorehnes  meriting 
special  description. 

m.  Neutral  Shore- 
lines. —  While  most 
of  the  world's  shore- 
lines have  resulted 
from  submergence  of 
land  areas  or  emer- 
gence of  subaqueous 
surfaces,  there  remain 
important  groups  of 
shorelines  whose  es- 
sential characteristics 
depend  on  causes  in- 
dependent of  either 
submergence  or  emer- 
gence. To  this  class  of  shorelines  I  propose  to  apply  the  term 
"neutral  shorelines."  Among  others,  the  class  will  include  the 
well-known  (a)  delta  shorelines  of  variable  form  and  extent. 
Where  the  current  of  a  river's  distributaries  strongly  predomi- 


FiG.  26.  —  Mississippi  Delta.     Atypical 
lobate  delta. 


188       TERMINOLOGY  AND   CLASSIFICATION  OF   SHORES 


nate  over  shore  currents  and  wave  attack,  the  delta  shoreHne 
will  be  of  the  "  lobate  "  type,  as  in  the  case  of  the  Mississippi 
delta  (Fig.  26).  If  shore  currents  or  possibly  wave  erosion,  or 
both,  have  a  marked  effect  in  shaping  most  of  the  accumulating 
delta  deposit,  but  the  river  along  one  principal  channel  con- 
tinues to  advance  its  mouth  into  the  lake  or  sea,  a  "  cuspate  " 

delta  shoreline  like  that  of  the 
Tiber  (Fig.  27)  will  result.  In 
case  either  shore  current  or  wave 
attack  sets  a  limit  to  delta  growth, 
even  at  the  mouths  of  distribu- 
taries, what  I  have  termed  an 
"  arcuate  "  delta  shoreline  may 
be  formed,  of  which  type  the 
Niger  delta  (Fig.  28)  seems  to  fur- 
nish a  good  example.  Interme- 
diate stages  between  these  several 
types,  or  combinations  of  two  or 
more  types  in  a  single  delta,  are 
frequently  encountered. 

Closely  related  to  delta  shore- 
lines are  (6)  alluvial  plain  shore- 
lines, and  (c)  outivash  plain  shore- 
lines, formed  where  the  broad 
alluvial  slope  at  the  base  of  a 
mountain  range  or  the  outwash 
plain  in  front  of  a  glacier  is  built 
forward  into  a  lake  or  the  sea. 
On  the  landward  side  of  such 
shorelines  the  topography  is  simi- 
lar in  many  respects  to  that  bor- 
dering the  coastal  plain  shoreline, 
and  the  same  may  be  true  of  the  immediate  offshore  zone. 
Farther  seaward  the  slope  would  normally  become  steeper,  like 
the  frontal  slope  of  a  delta,  (d)  Volcano  shorelines  of  more  or 
less  circular  pattern  are  formed  where  an  active  volcano,  pro- 
jecting above  the  water  surface,  builds  its  cone  upward  and  out- 
ward by  continued  addition  of  ejected  materials. 

A  very  important  group  of  neutral  shorelines  consists  of  (e) 
coral  reef  shorelines,  formed  when  coral  polyps  build  reefs  upward 


tc 

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/!^:  :"■  V  ■.~:!^^^- •'■■'.■'•  ■"■'■^^ 

/T:-.-.  ■ . .    .  .^j_r"/.  ■;  ."■  ■'■'■"r^. 

'%=-i;_^^^>^??^?=;^^     .■'•^% , 

N^.;  ■ . .  ■...■;  :^  ■  ''xlr^  ./'•/.^'^^I'll 

%• .;:  ;^5'"fl^i^^^!^;^;^ 

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rf  vyf 

rM^i^v.- 

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— s.^^^" 

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Fig.  27.  —  Delta  of  the  Tiber. 
A  cuspate  delta. 


NEUTRAL  SHORELINES 


189 


Fig.  28. 


Niger  Delta.     The  type  of 
arcuate  delta. 


from  a  submarine  floor  or  outward  from  the  margins  of  any  land 
area.  Whatever  the  influence  which  past  subsidence  of  the  sea- 
bottom  or  elevation  of  the  water  surface  may  have  exerted  upon 
the  particular  forms  assumed  by  coral  reefs,  does  not  affect  the 
fact  that  the  present  shorelines  of  the  reefs  owe  their  existence 
to  agencies  which  operate  independently  of  such  changes  of 
level.  The  reef  shore- 
lines do  not  mark  the 
contact  of  the  water 
surface  with  pre-ex- 
isting land  areas  or 
sea-bottoms,  but  with 
newly  made  land  in 
process  of  formation 
at  the  present  level. 
It  would  be  out  of 
place  to  enter  here 
into  a  discussion  of 
the  much  mooted 
coral  reef  problem. 
Those  desiring  to  pursue  this  subject  should  consult  the  writ- 
ings of  Davis^''^  and  Daly^"-,  wherein  discussions  of  the  earlier 
work  of  Darwin,  Dana,  Murray  and  Agassiz  will  be  found,  to- 
gether with  copious  references  to  the  extensive  literature  pub- 
lished by  other  investigators  of  the  problem.  Vaughan^"^  briefly 
discusses  different  theories  of  coral  reef  origin  in  a  short  paper 
published  in  1916. 

Another  important  group  of  shorelines  belonging  to  the  neutral 
class  consists  of  (/)  fault  shorelines.  The  best  discussion  of  this 
type  of  shorelines  is  contained  in  an  excellent  essay  by  Cotton, i**^ 
to  which  we  will  recur  on  a  later  page.  When  the  block  on  the 
downthrow  side  of  a  fault  is  so  far  depressed  as  to  permit  the 
waters  of  sea  or  lake  to  rest  against  the  fault  scarp,  we  have  the 
typical  fault  shoreline  (Fig.  29).  Cotton  describes  shorelines 
of  this  type  from  near  Wellington,  and  from  other  parts  of  New 
Zealand. 

Earlier  geological  literature  is  full  of  references  to  shorelines 
or  coasts  supposed  to  result  from  faulting.  Practically  every 
irregular  rocky  coast  has  been  explained  as  the  ragged,  broken 
edge  of  a  land  mass  bordering  a  down-dropped  segment  of  the 


190       TERMINOLOGY   AND   CLASSIFICATION  OF  SHORES 

earth's  crust.  Descriptions  of  these  coasts  abound  in  such  ex- 
pressions as  "  fractured  table-land  bordering  a  foundered  area," 
"  the  foundering  of  the  adjacent  ocean  bed,"  "  fractured  margins 
of  horsts,"  "  the  collapse  of  the  basin  of  the  Adriatic,"  and  "  shat- 
tered margin  of  the  continent."  Supported  by  the  authority 
of  men  like  Suess,  the  interpretation  of  irregular  coasts  as  the 


Fig.  29.  —  Initial  stage  of  a  fault  shoreline.     (Modified  after  Cotton.) 

ragged  edge  of  the  land  left  standing  when  the  adjacent  area 
foundered  beneath  the  sea,  gained  a  currency,  especially  among 
German  students^^^  out  of  all  proportion  to  its  merits.  It  is  now 
widely  recognized  that  most,  if  not  all,  of  these  extremely  ir- 
regular shores  are  Ijetter  explained  as  shorelines  of  submergence, 
unrelated  to  faulting.  Yet  not  a  few  writers  of  today,  including 
occasionally  a  trained  physiographer,  show  the  influence  of  Suess' 
teaching  by  invoking  the  "  shattering  and  foundering  "  theory  for 
coasts  like  those  of  Greece,  Dalmatia,  and  Norway.  Fault 
shorelines  exist;  but  so  far  as  described  by  critical  observers  on 
the  basis  of  competent  evidence,  they  do  not  exhibit  the  irregular 
pattern  of  the  coasts  just  mentioned.  Shorelines  of  submergence, 
on  the  contrary,  either  of  the  ria  or  fjord  type,  show  precisely 
those  characteristics  well  displayed  along  the  three  coasts  in 
question. 

IV.  Compound  Shorelines  are  those  which  are  prominently 
characterized  by  phenomena  normally  characteristic  of  at  least 
two  of  the  preceding  classes.  It  frequently  happens,  for  example, 
that  oscillations  in  the  level  of  land  or  sea  leave  a  shoreline  with 
a  variety  of  features,  some  of  which  resulted  from  sul)mergence, 
others  from  emergence.     This  is  the  case  with  the  shoreline  of 


COMPOUND  SHORELINES 


191 


North  Carolina  (Fig.  30), 
which  combines  the  drowned 
valleys  of  a  shoreline  of  sub- 
mergence with  the  offshore 
bar  of  a  shoreline  of  emer- 
gence in  such  manner  that 
it  is  difficult  to  decide  which 
set  of  features  is  more  promi- 
nent. We  can  therefore  most 
properly  speak  of  it  as  a  com- 
pound shoreline. 

In  the  formation  of  fault 
shorelines  it  may  well  happen 
that  the  block  on  the  up- 
throw side  of  the  fault  is 
itself  sufficiently  depressed  to 
permit  drowning  of  the  more 
deeply  cut  main  valleys  (Fig. 
31).  Such  cases  are  reported 
from  New  Zealand  by  Cot- 
ton^"^,  and  afford  a  very 
striking  example  of  compound  shorelines. 

The  term  compound  shoreline  should  be  employed  only  when 
there  is  a  very  marked  development  of  the  features  character- 
istic of  two  or  more  of  the  simpler  classes  of  shorelines.     Such 


Fig.  30.  —  Coast  of  North  Carolina, 
showing  one  type  of  compound 
shoreUne. 


Fig.  31.  —  Compound  shoieUne  due  to  faulting  and  partial  submergence  of 

upthrow  block. 


192        TERMINOLOGY  AND  CLASSIFICATION  OF   SHORES 

a  shoreline  as  that  of  eastern  Florida  would  be  classed  as  a 
shoreline  of  emergence,  notwithstanding  the  mild  indications 
of  submergence  presented  by  the  drowned  valleys. 

Stages  of  Shoreline  Development.  —  The  character  of  any 
shoreline  depends,  in  the  last  instance,  on  the  amount  of  work 
accomplished  by  marine  agents  upon  the  land  against  which  the 
water  surface  comes  to  rest ;  or,  in  other  words,  upon  the  stage  of 
shoreline  development.  Shorelines  of  submergence,  shorelines  of 
emergence,  neutral  shorelines,  and  compound  shorelines  of  all 
varieties  must  therefore  be  further  subdivided  into  groups  accord- 
ing as  they  are  in  the  initial,  young,  mature,  or  old  stage  of  de- 
velopment, each  group  having  its  own  peculiar  characteristics. 
What  these  characteristics  are  will  appear  at  some  length  in  the 
following  chapters. 

RESUME 

We  have  outlined  a  terminology  for  the  broader  topographic 
features  w'hich  characterize  the  margins  of  the  land  and  sea. 
These  features  include  four  zones,  known  as  the  coast,  shore, 
shore-face  and  offshore;  three  erosion  forms,  the  cliff,  bench,  and 
abrasion  platform;  and  three  deposits  called  the  beach,  veneer, 
and  continental  terrace.  All  of  these  features  are  not  invariably 
present,  for  we  have  already  observed  that  the  beach  may  be 
lacking,  and  it  will  appear  in  later  chapters  that  one  or  more 
of  the  other  features  mentioned  may  fail  to  be  developed  in 
special  cases.  Shorelines  have  been  classified  into  four  main 
groups:  shorelines  of  submergence,  shorehnes  of  emergence, 
neutral  shorelines,  and  compound  shorelines.  The  subdivisions 
of  each  class  have  been  briefly  considered,  and  some  examples 
cited.  We  are  now  prepared  to  study  the  development  of 
shores,  by  considering  first  the  development  of  the  shore  profile, 
after  which  the  shoreline  itself  will  be  treated. 


REFERENCES 

1.  Gulliver,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  152,  1899. 

2.  Wheeler,    W.    H.     The    Sea     Coast:     Destruction:     Littoral    Drift: 

Protection,  p.  vii,  London,  1902. 

3.  Gulliver,    F.    P.     Shoreline   Topography      Proc.    Amer.    Acad.   Arts 

and  Sciences.     XXXIV,  152,  1899. 


REFERENCES  193 

4.  Ratzel,  Fr.     Studien  iiber  Kilstensaum.     Berichte  K.  Sachs.  Gesells 

tier  Wissenschaften  zu  Leipzig,  Phil.-Hist.  Klasse.    LV,  202-211.  1903 

5.  Gulliver,  F.  P.     Shoreline  Topography.     Proc.  Amer  Acad.  Arts  and 

Sciences.     XXXIV,  152,  1899. 

6.  Barrell,    Joseph.     Criteria    for   the    Recognition    of    Ancient    Delta 

Deposits.     Bull.  Geol.  Soc.  Amer.     XXIII,  377-446,  1912 

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Review.    I,  20-47,  1916. 


CHAPTER  V 

DEVELOPMENT   OF  THE  SHORE   PROFILE 

SHORELINES  OF  SUBMERGENCE 

Advance  Summary.—  In  the  development  of  any  shoreline  there 
are  significant  and  systematic  changes  both  in  the  profile  and  in 
the  plan  of  the  land  margin.  These  changes  take  place  in  orderly 
sequence,  and  may  best  be  described  as  the  young,  mature,  and 
old  stages  of  a  cycle  of  shore  development.  We  shall  find,  how- 
ever, that  the  stages  of  shore  profile  development  and  the  stages 
of  shoreline  development  do  not  always  keep  pace  with  each 
other.  A  mature  shoreline  may  have  the  shore  profile  of  some 
of  its  parts  still  in  the  stage  of  youth;  and  it  very  commonly 
happens  that  a  young  shoreline  has  many  points  where  the 
shore  profile  is  mature.  It  will  be  desirable,  therefore,  to  discuss 
the  cycle  of  the  shore  profile  first,  and  later  to  consider  the  cycle 
of  the  shoreline  as  a  whole.  In  the  present  chapter  the  terms 
youth,  maturity,  and  old  age  refer  to  stages  of  profile  development 
only.  The  significant  features  of  the  profile  involve  all  four  of 
the  zones  adjacent  to  the  shoreline,  and  when  reference  is  made 
in  the  following  paragraphs  to  the  shore  profile,  it  should  be  under- 
stood to  include  not  only  the  shore  proper,  but  the  shoreface, 
offshore,  and  coast  as  well. 

It  has  been  deemed  advisable  to  discuss  first,  and  somewhat 
at  length,  profiles  characteristic  of  the  youth,  maturity  and  old 
age  of  shorelines  of  submergence.  Special  attention  is  given  to 
beach  profiles  and  to  their  constantly  shifting  forms,  matters  of 
vital  importance  in  many  problems  of  marine  engineering.  A 
stud}^  of  the  ultimate  stage  of  the  shore  profile  leads  logically  to 
a  consideration  of  the  theory  of  marine  planation.  This  theory 
is  discussed  fully  and  arguments  presented  to  support  the  opinion 
that  it  merits  a  greater  measure  of  confidence  than  most  stu- 
dents of  landforms  are  accustomed  to  accord  it.  The  marine 
and  fluvial  cycles  of  erosion  are  correlated,  their  essential  inde- 
pendence is  emphasized,  and  their  relative  importance  compared. 

199 


200 


DEVELOPMENT  OF  THE   SHORE   PROFILE 


X 
X 

Ah 


w 


c3 

a 


o 


INITIAL  STAGE 


201 


In  the  final  sections  of  the  chapter  the  fea- 
tures peciiHar  to  profiles  across  shorelines  of 
emergence,  neutral  shorelines,  and  compound 
shorelines  are  given  special  treatment. 

Initial  Stage.  —  The  initial  profile  at  right 
angles  to  a  shoreline  of  submergence  nor- 
mally indicates  comparatively  steep  slopes 
descending  rather  abruptly  into  the  water 
{a\  Fig.  32).  This  is  because  submergence 
ordinarily  permits  the  water  level  to  come 
to  rest  against  the  hill-sides  of  the  former 
land  area.  It  is  true  that  certain  excep- 
tions must  be  recognized.  If  the  land  area 
had  been  reduced  to  a  peneplane  surface 
before  submergence,  or  if  the  form  sub- 
merged were  a  young,  undissected  alluvial 
plain  or  other  similar  surface,  then  the  initial 
profile  will  resemble  that  normally  char- 
acteristic of  a  shoreline  of  emergence,  and 
the  history  of  development  will  be  that 
appropriate  for  such  a  profile.  Here  we 
deal  only  with  the  more  usual  case,  in  which 
submergence  permits  the  sea  to  come  to 
rest  against  the  irregular  and  comparatively 
pronounced  hill  slopes  of  a  submature, 
mature,  or  late  mature  land  mass. 

Waves  will  at  once  attack  the  land  at 
this  new  level,  their  vertical  zone  of  activity 
extending  from  a  short  distance  below  sea- 
level  to  a  short  distance  above;  because,  as 
we  have  already  seen,  the  forward  dashing 
crests  of  storm  waves  rise  some  feet  above 
mean  water  level,  while  the  vigor  of  wave 
activity  dies  out  very  rapidly  below  it. 
Although  the  waves  do  carry  on  a  milder 
erosive  activity  at  greater  depths,  the  attack 
near  the  surface  level  is  so  much  more 
vigorous  that  it  is  fair  to  liken  the  sea  to  a 
horizontal  saw  which  cuts  laterally  into  the 
land,  the  blade  of  the  saw  having  a  thick- 


202 


DEVELOPMENT  OF  THE  SHORE  PROFILE 


•3  ^ 

03 


YOUNG   STAGE  203 

ness  which  extends  from  a  few  feet  above  to  a  few  feet  below 
sea  level  and  being  armed  with  breaking  waves  for  teeth.  Wave 
erosion  soon  cuts  a  notch  in  the  edge  of  the  sloping  land,  and  thus 
destroys  the  initial  profile.  The  coarse  debris  resulting  from  this 
erosion  descends  the  underwater  slope  until  it  comes  to  rest  as 
a  submarine  talus,  where  the  water  is  deep  enough  to  render 
wave  agitation  mild  and  the  slope  is  gentle  enough  to  require 
much  agitation  for  the  ready  removal  of  coarse  material. 

Young  Stage.  —  Continued  wave  erosion  soon  pushes  the  notch 
so  far  into  the  land  that  the  unsupported  overhanging  rock  falls 
down  under  the  influence  of  the  forces  of  weathering,  inclucUng 
the  action  of  gravity  and  rain  wash  on  the  face  of  the  slope. 
Tliis  produces  the  wave-cut  cliff  {¥),  in  front  of  which  is  the  wave- 
cut  rock  terrace  called  the  bench  {h').  The  eroded  debris  will 
be  added  to  the  submarine  talus  (b^)  if  the  shoreface  slope  is 
steep  enough  and  the  water  deep  enough,  except  such  part  as  is 
ground  sufficiently  fine  to  be  widely  distributed  over  the  sea- 
bottom  far  offshore.  If  the  water  is  shallow  or  the  slope  gentle, 
a  shoreface  terrace  may  he  formed  at  this  time. 

In  the  cycle  of  stream  development,  the  longitudinal  profile 
of  the  young  stream  is  characterized  by  irregularities  which  had 
their  origin  in  the  initial  roughness  of  the  land  over  which  the 
stream  flowed,  or  in  the  unequal  erosion  of  alternate  belts  of  re- 
sistant and  non-resistant  rock.  Material  eroded  from  parts  of 
the  profile  exposed  to  vigorous  cutting  are  deposited  in  the  de- 
pressions of  the  profile  where  deep  water  is  found.  So  also  in  the 
young  shore  profile  we  have  irregularities  due  to  the  initial  rough- 
ness of  the  submerged  land  mass,  as  well  as  irregularities  in  both 
cliff  and  bench  {¥,  ¥)  due  to  unequal  resistance  of  the  rock  masses 
which  are  being  eroded;  and  the  material  torn  from  the  zones 
exposed  to  attack  are  deposited  in  the  depressions  of  the  profile 
where  deeper  and  quieter  waters  occur. 

As  Davis^  has  shown,  we  may  press  this  analogy  even  further 
with  profit.  In  the  typical  young  stream  the  water  movement  is 
vigorous  because  the  initial  slope  of  the  land  is  comparatively 
steep,  permitting  a  high  velocity;  transporting  and  eroding  power 
are  both  great,  but  the  transporting  power  is  far  more  than 
sufficient  to  remove  all  the  products  of  direct  erosion.  As  the 
stream  cuts  downward  the  valley  walls  are  undermined,  and 
weathering  causes  the  higher  portions  to  descend  into  the  stream 


204  DEVELOPMENT  OF  THE   SHORE   PROFILE 

channel.  But  even  the  addition  of  these  products  of  weathering 
does  not  over-tax  the  transporting  power  of  the  stream,  and  all 
the  debris  is  swept  down-valley  to  be  deposited  in  quieter  water 
below.  The  quantity  of  the  products  of  weathering  is  not  large, 
for  the  reason  that  since  the  stream  has  not  yet  cut  deeply  into 
the  land,  the  valley  walls  are  not  high  and  therefore  do  not  ex- 
pose any  considerable  area  to  the  forces  of  weathering.  Because 
the  rate  at  which  the  valley  walls  retreat,  due  to  the  forces  of 
weathering,  is  not  great  as  compared  with  the  rate  of  valley 
deepening  due  to  stream  erosion,  the  slope  of  the  valley  walls  is 
steep  and  may  be  vertical  or  even  over-hanging  in  places. 

Turning  our  attention  to  the  shore,  we  observe  precisely  anal- 
ogous conditions.  Along  the  typical  young  shoreline  of  sub- 
mergence the  wave  action  is  vigorous,  because  the  initial  slope  of 
the  coast  is  comparatively  steep,  permitting  large  waves  to  reach 
the  land;  transporting  power  and  eroding  power  are  both  great, 
but  the  transporting  power  is  far  more  than  sufficient  to  remove 
from  the  base  of  the  cliff  all  the  products  of  direct  erosion.  As 
the  waves  cut  inward  the  cliff  is  undermined,  and  weathering 
causes  the  higher  portions  to  descend  upon  the  marine  bench. 
But  even  the  addition  of  these  products  of  weathering  does  not 
over-tax  the  transporting  power  of  the  wave  currents,  and  all  the 
debris  is  swept  seaward  to  be  deposited  in  the  quieter  deep  water. 
The  quantity  of  the  products  of  weathering  is  not  large,  for  the 
reason  that  since  the  waves  have  not  yet  cut  far  into  the  land, 
the  marine  cliff  is  not  high  and  therefore  does  not  expose  any 
considerable  area  to  the  forces  of  weathering.  Because  the  rate 
at  which  the  face  of  the  cliff  retreats,  due  to  the  forces  of  weather- 
ing, is  not  great  as  compared  with  the  rate  of  backward  cutting 
due  to  wave  erosion,  the  slope  of  the  cliff  face  is  steep,  and  may 
be  vertical  or  even  over-hanging  in  places. 

A  later  stage  of  the  youth  of  the  shore  profile  shows  some  sig- 
nificant changes.  As  the  waves  cut  farther  into  the  land  their 
power  decreases  because  they  must  traverse  greater  and  greater 
stretches  of  shallow  water  over  the  broadening  marine  bench; 
just  as  the  stream  which  cuts  deeper  into  a  land  mass  suffers  loss 
of  erosive  power  because  the  water  must  flow  more  sluggishly  on 
gentler  and  gentler  gradients.  But  the  loss  of  wave  power 
comes  at  a  time  when  the  work  to  be  done  is  increasing,  for  the 
increased  height  of  the  cliff  enables  the  forces  of  weathering  to 


YOUNG  STAGE 


205 


-^3 


a 


13 


?S 


206  DEVELOPMENT  OF  THE   SHORE   PROFILE 

cast  a  larger  amount  of  debris  upon  the  marine  bench  below;  just 
as  the  higher  valley  walls  of  a  deepening  stream  shed  more  waste 
into  the  channel  at  the  very  time  the  stream  current  is  becoming 
more  sluggish  because  of  the  decreased  gradient.  In  both  cases 
the  work  to  be  done  increases  as  the  power  to  do  work  decreases. 
A  larger  proportion  of  the  weakening  wave  power  nuist  be  con- 
sumed in  transporting  the  increased  amount  of  debris  to  deep 
water  and  in  grinding  the  debris  finer  during  the  process  of 
transportation,  with  the  result  that  the  base  of  the  cliff  is  less 
and  less  vigorously  pushed  inland;  just  as  a  larger  proportion  of 
the  weakening  stream  power  must  be  used  up  in  transporting 
the  larger  volumes  of  waste  down-valley  with  the  result  that 
valley  deepening  is  still  further  diminished.  In  the  case  of  wave 
action,  weathering  now  has  the  opportunity  to  wear  back  the 
marine  cliff  to  a  more  gentle  slope  (c^),  which  corresponds  with 
the  more  gentle  slopes  of  the  valley  walls  in  the  similar  stage 
of  stream  development. 

Other  important  changes  remain  to  be  noted.  During  the 
appreciable  length  of  time  required  for  the  pushing  back  of  the 
cliff,  the  upland  surface  has  been  weathered  and  eroded  to  a 
lower  level  (c^).  Weathering  of  the  cliff  face  goes  on  rapidly 
enough  to  keep  pace  with  the  enfeebled  wave  cutting  at  the  base 
of  the  cliff,  so  that  there  is  no  longer  a  prominent  notch  at  the 
level  of  wave  erosion.  The  accumulation  of  the  debris  swept 
seaward  from  the  marine  bench  by  wave  and  possibly  other 
currents  has  resulted  in  the  formation  of  a  shoreface  terrace  {d) 
whose  top  surface  is  delicately  adjusted  to  continue  the  slight 
seaward  inclination  of  the  marine  bench  (c^). 

Still  more  important  is  the  fact  that  the  marine  bench  main- 
tains its  seaward  inclination,  and  is  therefore  lower  at  its  outer 
margin  than  it  was  at  that  same  locality  in  an  earlier  stage  of 
development.  Thus  the  bench  at  c-  has  been  lowered  from  the 
position  h^.  There  should  be  no  difficulty  in  understanding  this 
important  change,  and  its  causes  and  consequences.  Waves 
continue  to  traverse  the  marine  bench,  and  as  the  depth  of  the 
bench  is  not  yet  great  enough  to  place  it  beyond  the  reach  of 
wave  action,  it  must  suffer  some  erosion.  The  very  fact  that 
waves  are  weakened  as  they  cross  the  bench  towarrl  the  cliff 
proves  that  they  have  lost  energy  by  expending  it  on  the  bottom. 
The  debris  weathered  from  the  face  of  the  cliff  and  eroded  from 


YOUNG  STAGE 


207 


'S 

>> 

'& 

^ 

tS 

"3 

c3 

> 

O 

o 

O 

u 

-2   >^ 


3-^ 


208  DEVELOPMENT  OF  THE   SHORE  PROFILE 

its  base  is  dragged  across  the  marine  bench  by  wave  currents, 
possibly  aided  by  other  currents,  to  be  built  into  the  shoreface 
terrace  or  moved  into  deeper  water;  and  the  long-continued 
action  of  this  "  marine  sandpaper  "  must  grind  the  surface  of  the 
bench  ever  lower  and  lower.  As  the  outer  part  of  the  bench  has 
been  made  longest  and  therefore  exposed  to  continuous  abrasion 
for  the  longest  time,  it  is  worn  lower  than  the  parts  further  land- 
ward.    Thus  the  bench  keeps  its  seaward  inclination. 

The  effects  of  the  seaward  inclination  of  the  marine  bench  are 
all-important.  We  have  seen  that  waves  tend  to  break  when  the 
depth  of  the  water  equals  the  height  of  the  wave;  hence  the 
deeper  the  water  the  larger  the  waves  which  can  traverse  it. 
Progressive  lowering  of  the  marine  bench  therefore  means  the 
continuous  admission  of  large  waves  farther  and  farther  across 
its  surface.  Were  it  not  for  this  lowering,  a  shallow,  horizontal 
bench  would  greatly  reduce  the  size  and  power  of  the  waves 
which  reached  the  cliff.  While  this  would  not  completely  stop 
cliff  erosion,  as  has  sometimes  been  assumed,  it  would  enormously 
retard  it.  The  seaward  inclination  of  the  bench  greatly  facili- 
tates the  removal  of  debris  into  deep  water;  for  as  we  found  from 
our  study  of  wave  action,  if  oscillatory  waves  produce  equal  im- 
pulses alternately  landward  and  seaward,  debris  on  an  inclined 
sea-bottom  must  travel  down  the  slope,  whereas  on  a  horizontal 
bottom  it  might  remain  in  one  place  indefinitely.  Effective 
removal  of  debris  prevents  it  from  protecting  the  cliff,  and  per- 
mits the  waves  to  devote  a  greater  proportion  of  their  energy 
to  cliff  erosion.  Thus  in  a  second  way  the  progressive  lowering 
of  the  marine  bench  in  such  manner  as  to  produce  an  inclined 
surface,  greatly  facilitates  the  recession  of  the  shoreline  under 
wave  attack.  A  third  important  effect  of  the  inclined  bench  is 
to  raise  the  level  of  effective  wave  attack  at  the  base  of  the  cliff. 
We  have  observed  in  preceding  chapters  that  winds  blowing 
toward  a  steep  coast  with  deep  water  offshore  do  not  raise  the 
water  level  appreciably,  but  that  where  the  water  is  shallow  its 
entire  mass  may  receive  a  landward  motion  and  thus  pile  up 
against  the  shore;  that  both  oscillatory  waves  and  waves  of 
translation  coming  onshore  raise  the  water  level,  waves  of  trans- 
lation most  effectively;  that  oscillatory  waves  change  to  waves 
of  translation  on  a  gradually  shelving  bottom;  and  finally, 
that  tidal  and  other  currents  moving  in  upon  such  an  inclined 


YOUNG  STAGE 


209 


&H 


u 


210  DEVELOPMENT  OF  THE   SHORE   PROFILE 

slope  raise  the  water  level  more  effectively  than  when  they  im- 
pinge upon  a  steep  slope  which  descends  rapidly  to  deep  water. 
All  of  these  factors  co-operate  to  raise  the  level  of  wave  attack, 
especially  during  storms,  to  a  slightly  higher  position  as  the 
shoreline  is  pushed  inland.  On  the  other  hand,  the  development 
of  strong  waves  of  translation  on  the  shallowing  bottom  during 
sto^i'ms  may  move  debris  landward  temporarily,  thereby  delaying 
its  removal  from  the  marine  bench  into  deep  water,  and  so  re- 
tarding cliff  recession  for  a  time. 

The  notch  at  the  base  of  the  marine  cliff  is  a  measure  of  the 
ratio  between  wave  erosion  and  weathering  (including  the  ac- 
tion of  gravity) .  When  wave  erosion  is  much  the  more  vigorous, 
a  pronounced  notch  occurs;  when  erosion  exceeds  weathering 
but  slightly,  the  notch  is  only  faintly  developed,  and  if  weathering 
is  able  to  keep  pace  with  erosion  there  will  be  no  notch.  Un- 
consolidated materials  are  quickly  pulled  down  upon  the  marine 
bench  by  the  action  of  gravity,  which  may  be  regarded  as  a  very 
important  element  of  weathering,  since  it  is  most  efficient  in 
promoting  the  disintegration  of  rock  masses.  As  a  result,  ero- 
sion cannot  gain  on  weathering  sufficiently  to  produce  a  notch  in 
sand  cliffs  and  other  unconsolidated  material,  even  in  the  earliest 
stages  of  cliff  erosion;  whereas  rocky  coasts  may  possess  good 
notches  in  early  youth,  faint  notches  in  late  youth,  and  none  in 
maturity  when  weathering  and  erosion  are  delicately  balanced. 

On  tidal  shores,  especially  where  the  range  of  the  tides  is  great, 
account  must  be  taken  of  the  varying  water  level.  In  the  initial 
stage  the  vertical  extent  of  the  notch  may  be  increased  because  of 
wave  erosion  throughout  the  whole  extent  of  the  tidal  range. 
But  early  in  youth  it  will  be  found  that  the  notch  is  developed 
at  the  high  tide  level.  Larger  and  more  vigorous  waves  reach 
the  coast  in  the  deeper  water  of  high  tide,  and  the  cliff  is  pushed 
in  more  vigorously  at  that  higher  level.  The  waves  at  low  tide 
are  left  to  expend  their  force  on  the  shelving  marine  bench,  and 
thus  to  assist  in  deepening  its  seaward  portion.  The  general 
relations  of  the  different  topographic  elements  along  the  shore 
are  not  greatly  different  from  those  which  would  obtain  if  the 
high  tide  level  were  the  mean  water  level  of  a  tideless  sea.  Some 
minor  differences  will  be  noted  as  occasion  demands. 

Mature  Stage.  —  The  essential  feature  of  maturity  in  the 
development  of  the  shore  profile  is  a  condition  of  approximate 


MATURE  STAGE 


211 


equilibrium  between  erosion,  weathering,  and  transportation. 
In  other  words,  the  profile  of  maturity  is,  as  in  the  case  of  the 
mature  stream,  a  profile  of  equilibrium-.  During  j^outh  the 
power  of  the  waves  to  do  work  is  far  in  excess  of  the  work  to  be 
done.  But  as  the  development  progresses  the  work  to  be  done 
constantly  increases,  while  the  power  to  do  work  ever  diminishes. 
There  must  come  a  time  when  the  two  are  nicely  balanced  and 
equilibrium  is  established.  This  time  ushers  in  the  stage  of 
maturity. 

The  essential  nature  of  the  shore  profile  of  equilibrium  may  be 
better  appreciated  from  an  inspection  of  the  accompanying 
diagram  (Fig.  33).  Where  the  cliff  profile  is  steep  (c)  and  much 
debris  is  shed  into  the  water,  the  waves  require  a  comparatively 
steep  subaqueous  slope  in  order  that  with  the  effective  aid  of 
gravity  Lhey  may  be  able  to  remove  the  large  amount  of  debris 


Fig.  33.  —  Successive  profiles  of  equilibrium  on  a  retrograding  shore. 


offered  to  them.  With  cliffs  of  progressively  decreasing  steepness 
(c'  and  c"),  more  gently  inclined  subaqueous  slopes  will  permit 
that  nice  balance  between  the  amount  of  work  required  to  re- 
move the  diminished  quantity  of  debris  and  the  ability  of  the 
waves  to  do  removal  work,  which  we  call  "  equilibrium."  The 
subaqueous  profile  is  steepest  near  the  land  where  the  debris  is 
coarsest  and  most  abundant;  and  progressively  more  gentle 
farther  seaward  where  the  debris  has  been  ground  finer  and  re- 
duced in  volume  by  the  removal  of  part  in  suspension.  At  every 
point  the  slope  is  precisely  of  the  steepness  required  to  enable  the 
amount  of  wave  energy  there  developed  to  dispose  of  the  volume 
and  size  of  debris  there  in  transit.  Examples  of  actual  profiles 
of  equihbrium  are  shown  in  Figure  34. 

Let  us  imagine  that  the  profile  cU-cP  (Fig.  35)  is  the  shore  pro- 
file of  equilibrium,  and  verify  the  condition  of  equilibrium  by 


212 


DEVELOPMENT  OF  THE  SHORE  PROFILE 


noting  the  consequences  which  must  arise  if  we  disturb  any  part 
of  that  profile.  By  assumption  the  erosion  at  the  base  of  the 
cHff  is  just  sufficient  to  supply  the  amount  of  debris  which,  added 
to  the  material  contributed  by  the  weathering  of  the  cliff  face, 
will  provide  the  wave  currents  with  the  exact  amount  of  ma- 
terial they  can  transport  across  the  marine  bench  and  shoreface 


0  Miles                                                           10 

2( 

^^"S^.. 

^^~"T~*— sv- — 1»  . 

•     • 

•                         • 

West  Coast  Madagascar  Lat.  S.  18°  53'  to  19°  03' 

— • 

^ 

\ 

\ 

\ 

n 

0  Fathoms 
10 

20 
30 
40 
50 


Southeast  Coast  Madagascar  *  •        »  '  J  5<1 

Between  C.Ranovalona  and  Galleon  Bay  Lat.  S.  25°05' 

Fig.  34.  —  Profiles  of  equilibrium  off  the  Madagascar  coast  as  plotted  from 
charts  by  Barrell.  Note  the  striking  difference  between  the  profile  of 
the  protected  west  coast  and  that  of  the  exposed  southeast  coast . 

terrace  to  the  front  slope  of  the  latter.  Now,  if  we  imagine 
the  cliff  (d^)  to  weather  more  rapidly  for  any  cause,  this  will 
mean  an  added  accumulation  of  debris  at  the  base  of  the  cliff. 
The  waves  will  have  more  material  to  transport  and  therefore 
less  energy  left  to  expend  in  erosion.  Hence  the  base  of  the  cliff 
is  pushed  back  less  rapidly  than  normally.  But  since  the  top 
of  the  cliff  has  weathered  back  more  rapidly  than  usual,  the 
ultimate  result  is  a  gentler  slope  for  the  cliff  face.  On  the  gentler 
slope  weathering  proceeds  less  rapidly  than  formerly,  until  the 
waves  get  rid  of  their  excess  burden  and  renew  their  erosion  at 
the  base  of  the  cliff,  thereby  steepening  it  until  weathering  is 
once  more  normally  adjusted  to  the  other  forces  and  equilibrium 


V^' 


MATURE  STAGE 

is  re-established.  In  a  similar  manner,  if 
we  disturb  the  equilibrium  by  increasing 
the  wave  erosion,  this  will  mean  more 
eroded  material  and  products  of  weathering 
to  be  transported,  wave  currents  will  be 
overburdened,  debris  will  accumulate  un- 
duly on  the  marine  bench,  thereby  shallow- 
ing the  water  and  decreasing  the  size  of  the 
waves  which  can  reach  the  cliff  base,  thus 
reducing  erosion  until  equilibrium  is  again 
restored.  Increase  of  transporting  power 
would  sweep  the  marine  bench  clean  and 
allow  waves  to  deepen  it  more  effectively, 
thereby  admitting  larger  waves  to  the  cliff 
base  to  produce  greater  erosion,  and  so  in- 
creasing the  material  to  be  moved  until  the 
transporting  power  of  the  waves  was  again 
balanced  by  the  amount  of  material  requir- 
ing removal. 

As  in  a  mature  river  the  equilibrium  is 
never  absolutely  perfect,  but  rather  an 
ideal  condition  which  the  stream  ever 
strives  to  attain  and  does  succeed  in  ap- 
proximating very  closely;  so  at  the  shore, 
where  the  variation  in  wave  attack  is  far 
more  irregular  than  stream  volume  and 
velocity,  the  equilibrium  of  maturity  is  only 
approximate.  Each  set  of  waves  endeavors 
to  establish  a  profile  of  equilibrium  suited 
to  its  own  needs,  but  seldom  succeeds  be- 
fore another  set  of  waves  begins  working 
toward  a  somewhat  different  profile.  Fortu- 
nately, the  small  waves  work  so  slowly  as 
to  effect  no  profound  changes  between  times 
of  vigorous  wave  action,  while  the  attack 
of  storm  waves  at  a  given  point  is  sufficiently 
similar  at  different  times  to  produce  similar 
effects.  There  is,  therefore,  a  certain  char- 
acteristic profile  of  equilibrium  for  a  given 
locality,    notwithstanding    the    fact    that 


213 


> 


^ 


C^    r 


214  DEVELOPMENT  OF  THE  SHORE  PROFILE 

minor  variations  in  the  forces  there  at  work  will  produce  local 
changes  which  tend  to  confuse  the  student  of  shore  forms.  Let 
us  first  note  the  broader  features  of  the  mature  profile,  and  then 
consider  some  of  the  more  variable  minor  features. 

In  Figure  35  the  profile  d^-d^  is  that  of  maturity.  Comparing 
it  with  c^c*,  the  profile  of  late  youth,  we  note  certain  significant 
differences.  The  cliff  d^  has  weathered  back  to  a  more  gentle  slope 
than  in  c^,  because  wave  attack  is  more  feeble  when  the  waves 
must  traverse  a  broader  marine  bench  which  is  encumbered,  as 
we  shall  see,  by  more  or  less  debris.  As  erosion  carries  the  cliff 
farther  and  farther  inland  it  will  from  time  to  time  occupy  posi- 
tions on  the  landward-sloping  sides  of  hills,  in  which  positions  the 
cliff  decreases  in  height  as  it  advances  into  the  land  (Plate  VII) . 
The  presence  of  such  cliffs  of  decreasing  altitude  along  a  coast 
implies  considerable  wave  erosion  in  the  past.  The  upland  (rf^) 
has  worn  down  to  a  lower  level  during  the  time  required  for 
the  cliff  to  retreat  from  c^  to  fP.  As  should  be  expected,  the 
marine  bench  has  likewise  been  worn  lower  at  the  same  time  that 
it  has  been  extended  inland;  but  it  should  be  observed  that  al- 
though the  cliff  retreated  twice  as  far  from  c^  to  fP  as  from  b^  to  c^, 
the  outer  part  of  the  marine  bench  has  not  been  lowered  in  pro- 
portion. This  is  because  the  waves  act  more  feebly  with  in- 
creasing depth,  and  because  the  bench  is  more  protected  by  debris 
than  formerly.  In  consequence  of  the  rapid  decrease  in  wave 
power  with  increase  in  depth,  the  shoreward  portion  of  the  bench 
has  a  steeper  slope  than  the  portion  in  deeper  water;  or,  in  other 
words,  the  profile  of  the  bench  is  faintly  concave  upward.  A 
notable  extension  of  the  shoreface  terrace  (r/i)  is  apparent,  the 
front  of  the  terrace  being  convex  upward.  The  compound  pro- 
file of  the  bench  and  shoreface  terrace  combined  is  therefore 
roughly  sigmoid,  faintly  concave  upward  near  the  landward  end 
and  convex  at  the  seaward  end. 

The  most  important  feature  of  maturity  is  the  accumulation 
of  debris  on  the  marine  bench  to  form  a  beach.  During  youth 
the  vigorous  wave  action  sweeps  the  products  of  weathering  and 
erosion  into  deep  water  so  rapidly  that  there  may  be  no  conspic- 
uous deposits  of  waste  on  the  bench  most  of  the  time.  In  matur- 
ity, however,  the  journey  from  the  base  of  the  cliff  to  deep  water 
is  so  long,  and  wave  action  over  much  of  the  distance  is  so 
moderate,  that  any  given  moment  may  -witness  a  considerable 


MATURE  STAGE  215 

quantity  of  debris  in  transit  across  the  bench.  Under  normal 
conditions  this  material  is  not  of  appreciable  depth,  and  its  inter- 
mittent seaward  movement  serves  to  reduce  the  size  of  its  com- 
ponent parts  and  to  lower  the  level  of  the  bench,  by  friction 
amongst  the  particles  themselves  and  upon  the  bench  surface. 
It  is  this  beach  deposit  which  undergoes  the  most  sudden  and 
repeated  changes  which  characterize  the  shore  and  shoreface 
zones,  and  we  may  now  turn  our  attention  for  a  few  moments 
to  these  changes  and  their  causes. 

The  Beach.  —  In  the  first  place,  it  must  be  borne  in  mind  that 
the  beach  is  merely  a  temporary  deposit,  slowly  making  its  way 
to  deeper  water.  If  the  various  shore  processes  were  perfectly 
uniform  in  their  actions  and  always  nicel}^  adjusted  to  each  other, 
the  thickness  of  the  deposit  and  its  surface  profile  would  remain 
essentially  the  same,  while  the  component  particles  in  the  de- 
posit would  constantly  migrate  seaward  and  be  as  constantly 
replaced  bj-  new  material  weathered  and  eroded  from  the  marine 
cliff  and  bench.  But  the  forces  are  variable,  both  in  character 
and  intensity.  Oscillatory  waves  may  be  replaced  by  waves  of 
translation  at  irregular  intervals;  the  undertow  varies  in  volume 
and  velocity  and  is  modified  by  other  currents;  waves  vary  in 
size  from  day  to  day,  and  the  storm  waves  of  one  season  are  more 
powerful  than  those  of  another  season.  All  of  these  changes, 
and  others  that  might  be  enumerated,  distui'b  the  equilibrium 
which  would  otherwise  exist,  and  the  beach  deposit  responds 
quickly  to  these  disturl)ances.  At  one  time  wave  erosion  at 
the  base  of  the  cliff  supplies  material  faster  than  it  can  be  trans- 
ported, and  the  beach  deposit  accumulates  to  a  greater  depth  than 
usual.  At  another  time  waves  fail  to  reach  the  cliff  base  for  a 
long  period,  and  the  beach  wastes  away  because  the  loss  it  suffers 
from  continual  attrition  and  removal  under  the  influence  of 
small  waves  is  not  made  good  by  new  supplies  of  debris.  Again, 
waves  of  translation  drive  in  much  material  from  the  shoreface 
terrace  and  even  from  the  deeper  water  beyond,  piling  it  upon 
the  normal  beach  deposit  and  thereby  greatly  augmenting  its 
thickness.  Or  storm  waves  accompanied  by  a  vigorous  under- 
tow may  sweep  the  entire  beach  from  the  marine  bench,  leaving 
the  bare,  solid  rock  exposed  over  extensive  areas. 

The  factors  involved  in  shore  processes  are  so  numerous,  and 
their  variations  are  so  difficult  to  trace,  that  it  is  often  im- 


216  DEVELOPMENT  OF  THE  SHORE  PROFILE 

possible  to  ascertain  just  what  disturbance  of  former  conditions 
is  responsible  for  a  given  change  in  the  beach,  As  Hunt^  has 
said:  "  A  beach  may  resist  the  sea  for  years,  yet  in  a  few  hours  it 
may  be  stripped  bare  to  the  olid  rock.  Shells  may  be  covering 
the  bottom  a  mile  offshore,  undisturbed  by  onshore  gales;  a 
storm,  with  wind  and  waves  apparently  much  the  same  as  usual, 
may  sweep  them  all  onshore.  One  beach  will  be  invariably 
kept  clear  of  shells  which  will  be  found  offshore,  while  another 
beach  will  have  a  constant  supply,  and  for  no  obvious 
reason." 

We  may  gain  some  appreciation  of  the  extent  of  the  above- 
mentioned   changes  in  the  beach   deposit  from   the  published 
reports  of  competent   observers.     Reference  has  already  been 
made  in  an  earlier  chapter  to  the  shingle  and  chalk  ballast  driven 
in  upon  the  beaches  between  Tyne  and  Hartlepool,  England, 
from  points  7  to  10  miles  offshore.'*     Along  the  coast  of  Algeria 
the  waves  cast  large  quantities  of  sand  upon  the  beach,  burying 
the  roadway  along  the  shore  for  considerable  distances,  a  phenom- 
enon well  described   by  Fischer.^     At  one   point  on   the  Irish 
coast,  according  to  Kinahan,'^  a  beach  200  yards  wide  was  built 
in  front  of  a  marine  cliff  during  the  spring  of  1876,  at  a  point 
where  there  was  deep  water  the  previous  winter.     The  presence 
of  a  beach  deposit  along  a  shore  for  much  of  the  time  is  apt  to 
give  one  a  false  idea  of  its  depth  and  stability.     Thus  many 
visitors  to  the  beaches  of  the  Atlantic  coast  find  it  difficult  to 
realize  that  a  single  storm  will  often  strip  bare  the  underlying 
rock  or  expose  buried  peat  deposits  at  places  where  they  never 
see  anything  but  an  apparently  inexhaustible  store  of  beach 
sand.     Hunt^  refers  to  a  case  in  which  the  eastern  half  of  the 
shore   at   Blackpool,   near  Dartmouth,    England,   was  stripped 
bare  of  its  beach  sands,  for  the  only  time  in  twenty-five  years  so 
far  as  was  known.     The  bathing  beach  at  Babbicombe  was, 
according  to  this  same  author,   so  completely  removed  by  a 
single  storm  that  the  place  looked  "  as  unlike  a  bathing-cove  as 
any  place  can  be."     The  southern  coast  of  England  is  well  known 
for  its  extensive  beach  deposits;    yet   God  wen  Austen^  writes: 
"  I  have  seen,  at  one  time  or  another,  nearly  every  portion  of 
our  south  coast  in  the  condition  of  bare  rock  without  sand  or 
shingle.  .  .     Bars,  sand- and  shingle-banks  .  .  .  are  all  subject  to 
change  of  form  and  to  removal,  but  they  speedily  collect  again." 


MATURE  STAGE  217 

During  the  first  few  hours  of  a  gale  enough  material  may  be  re- 
moved from  the  shoreface  to  deepen  the  water  there  from  5  to  10 
feet,  especially  where  a  sea  wall  helps  to  concentrate  the  wave 
energy  along  a  narrow  zone.^  Along  the  Chesil  Bank  between 
Abbotsbury  and  Portland,  Coode^"  estimated  that  a  single  storm 
removed  3,763,300  tons  of  shingle  from  the  beach;  and  during 
another  storm  4,500,000  tons  of  the  shingle  were  scoured  out, 
three-fourths  of  which  was  moved  back  after  the  gale  ceased. ''• 
Pendleton^^  states  that  the  shoreline  of  the  beach  along  the 
southern  coast  of  Long  Island  has  varied  temporarily  back  and 
forth  200  to  300  feet,  due  to  storms. 

Beach  Profile  of  Equilibrium.  —  During  all  the  temporary 
changes  referred  to  above,  the  profile  of  equilibrium  is  maintained 
in  as  great  perfection  as  the  rapidly  varying  conditions  will 
permit.  Whether  developed  on  the  rock  bench  or  on  a  thick 
overlying  beach,  the  profile  is  concave  upward.  The  concavity 
continues,  with  increasingly  steep  slope,  above  the  normal  water 
level,  because  the  swash  of  the  waves  sweeps  debris  up  the  beach 
and  deposits  it  in  such  manner  as  to  maintain  the  necessary  equi- 
librium between  the  onshore  and  offshore  forces.  Near  the  water 
line  both  the  swash  and  the  l^ackwash  of  the  waves  have  large 
volume  and  high  velocity,  and  debris  is  swept  back  and  forth  on 
a  fairly  gentle  slope.  Farther  up  the  beach  the  swash  suffers  loss 
of  velocity  because  of  increasing  friction  and  the  constant  down- 
ward pull  of  gravity;  and  loss  of  volume  because  much  water  sinks 
out  of  sight  into  the  interstices  between  the  beach  pebbles  and 
sand.  Consequently  debris  is  deposited  at  the  higher  level  of  the 
beach  and  the  backwash  is  too  weak  to  return  it  to  the  sea.  But 
the  very  act  of  deposition  steepens  the  upper  part  of  the  slope, 
thereby  increasing  the  effectiveness  of  the  pull  of  gravity  upon 
the  debris,  so  that  a  small  backwash  can  the  more  readily  carry 
material  back  down  the  slope.  Equilibrium  is  attained  when 
the  slope  is  so  steep  that  the  backwash  aided  by  gravity  can  just 
return  all  the  material  which  the  larger  swash  can  drive  upward 
against  the  pull  of  gravity.  In  general,  it  may  be  said  that  in 
maturity  the  beach  profile  both  in  the  shore  and  shoreface  zones, 
is  either  nicely  adjusted  to  the  conditions  imposed  by  a  set  of 
waves  which  have  been  operating  for  some  time,  or  is  rapidly 
undergoing  adjustment  to  a  new  set  of  waves  which  differ  from 
those  previously  operating.     Let  us  note  some  of  the  changes 


218 


DEVELOPMENT  OF  THE  SHORE  PROFILE 


in  the  beach  profile  which  result 
from  these  adjustments  to  varying 
conditions. 

Imagine  a  mature  shore  profile 
(aaa,   Fig,   36)    in   which    a  thin 
beach  deposit  covers  the  marine 
bench  and  is  continued  seaward 
by   the   shoreface   terrace.     First 
let   us   suppose   that   a   series  of 
g    oscillatory     waves     encountering 
■3    the  seaward  edge  of  the  terrace 
5   are    partially    transformed    into 
S    waves  of  translation.     The  waves 
of  translation  will  then  drive  the 
bottom"debris  landward  and  bank 
^    it  up  against  the  base  of  the  cliff, 
2    building  the   beach    deposit   for- 
ward and  making  its  front  of  such 
steepness  that  gravity  plus  under- 
tow will  just  balance  the  tendency 
of  the  shoreward   component  to 
carry  material  up  the  slope.     But 
the  taking  of  material  from  the 
bottom  deepens   the  water,   and 
deepening  water  is  more  and  more 
unfavorable  to  the  development 
of    waves    of    translation.     The 
waves  retain  more  of  their  oscilla- 
tory character  than  formerly;  and 
with    the    more    sudden    descent 
into  deep  water  in  front  of  the 
new   deposit    the    undertow    be- 
comes more  effective,  finally  over- 
coming   further    efforts     toward 
landward     transportation.        We 
will   then    have    the   profile    666, 
which  is  the  profile  of  equilibrium 
under  the  new  conditions. 

Now  let  us  imagine  that  this 
new   profile   is   subjected   to   the 


MATURE   STAGE  219 

action  of  smaller  oscillatory  waves,  in  which  the  offshore  com- 
ponents (backward  oscillation  +  gravity  on  the  steep  slope  -\- 
undertow)  are  in  excess  of  the  shoreward  components.  Material 
will  be  eroded  from  the  upper  part  of  the  deposit  and  carried 
seaward.  But  the  undertow  associated  with  these  smaller 
waves  does  not  possess  a  great  transporting  power.  Conse- 
quently much  of  the  seaward  moving  debris  will  be  quicldy 
dropped,  thus  building  up  the  bottom  to  a  higher  level  and  de- 
creasing the  water  depth.  The  effect  of  this  is  to  restrict  the 
undertow  within  smaller  limits  and  so  to  increase  its  velocity  until 
it  is  able  to  transport  all  the  material  eroded  by  the  waves. 
Thus  equilibrium  between  the  various  factors  is  once  more  per- 
fected. Since  the  load  of  moving  debris  is  in  equilibrium  with 
the  transporting  currents  at  a  higher  level  than  before,  a  new 
shoreface  terrace  ccc  builds  forward  over  the  former  bench,  and 
possibly  over  the  older  terrace. 

Finally,  let  us  imagine  that  a  series  of  great  storm  waves,  ac- 
companied by  a  vigorous  undertow,  attacks  the  shore  under 
consideration.  Erosive  power  is  great  enough  to  cut  into  the 
beach  deposit  and  remove  it,  and  possibly  to  attack  the  cliff 
itself;  and  the  seaward  currents  along  the  bottom  are  more  than 
strong  enough,  at  the  higher  level  of  the  profile  cc,  to  transport 
all  debris.  They  therefore  erode  the  bottom,  deepening  the 
water,  and  thus  decreasing  their  velocity  until  they  are  just  able 
to  transport  the  material  delivered  to  them.  It  may  well  be 
that  this  new  equilibrium  is  not  reached  until  the  marine  bench 
is  swept  clean  and  the  profile  of  bench  and  terrace  reduced  to 
the  line  ddd. 

Other  changes  in  the  profile  of  the  beach  must  result  from  other 
variations  in  the  on-  and  offshore  forces.  An  offshore  wind  may 
cause  a  landward  bottom  current,  as  we  have  already  seen,  and 
this,  aided  by  wave  agitation,  builds  the  beach  forward  until  the 
front  slope  is  so  steep  and  the  water  so  deep  that  equilibrium  is 
restored.  An  onshore  wind  may  develop  such  a  vigorous  under- 
tow that  the  bench  will  be  stripped  of  much  of  its  deposits  be- 
fore equilibrium  is  again  reached.  This  would  explain  Coode's^^ 
observation  that  after  offshore  winds  the  slope  of  a  shingle  beach 
is  1  in  3|  or  4;  whereas  after  heavy  onshore  winds  the  slope  is 
only  1  in  9  or  9|.  If  the  supply  of  waste  from  the  cliff  is  stopped 
for  any  reason,  the  beach  will  be  removed  and  the  bench  lowered 


220  DEVELOPMENT  OF  THE  SHORE  PROFILE 

to  a  new  profile.  On  the  other  hand,  if  a  change  in  the  character 
of  cUff  material  should  result  in  a  more  rapid  supply  of  debris, 
the  seaward  currents  will  be  too  weak  to  transport  all  the  debris 
until  deposition  has  shallowed  the  water  and  thereby  increased 
the  current  velocity;  or,  as  Fenneman"  has  expressed  it:  "  If  the 
supply  of  material  be  suddenly  increased,  a  smaller  shelf  will 
grow  from  shore  on  the  surface  of  the  >  older,  for  the  reason  that 
the  new  load,  being  greater,  is  in  equilibrium  with  the  currents  at 
a  higher  level  than  before."  In  all  of  these  and  other  similar 
changes,  the  profile  of  equilibrium  is  either  maintained  or  quickly 
restored. 

From  what  has  been  said  in  the  preceding  paragraphs,  it  is 
evident  that  man  has  the  power  to  retard  cliff  erosion  if  he  can 
deposit  a  sufficient  amount  of  debris  upon  the  shore  to  overload 
the  waves  and  cause  them  to  establish  a  profile  of  equilibrium 
which  does  not  touch  the  bare  rock  of  bench  or  cliff.  On  the 
other  hand,  man  may  accelerate  cliff  erosion  by  removing  sand  or 
shingle  from  the  beach,  thereby  causing  the  waves  to  expend 
their  excess  energy  in  retrograding  the  shoreline  until  a  new 
profile  of  equilibrium  is  established.  Legal  authorities  have  taken 
cognizance  of  this  latter  possibility  in  a  number  of  cases.  Thus 
the  British  Board  of  Trade  has  repeatedly  prohibited  the  removal 
of  beach  material  from  shores  where  it  was  clear  that  such  re- 
moval would  be  injurious  to  the  coast.  In  an  action  brought  by 
the  Attorney  General  against  a  certain  lord  who  asserted  his 
right  to  remove  shingle  from  his  own  shores,  it  was  held  that  it 
was  the  duty  of  the  Crown  to  protect  the  realms  from  inroads  of 
the  sea  by  maintaining  the  beaches  in  their  natural  condition; 
and  an  injunction  was  granted  restraining  any  further  removal. ^^ 
When  the  removal  of  shhigle  from  the  beach  at  Spurn  Point, 
England,  for  road  mending  and  concrete,  was  stopped,  the 
erosion  of  the  cliffs  diminished  one-half.^^  On  the  Prussian 
shores  the  taking  of  stones  from  the  beach  is  "  polizeilich  ver- 
boten." 

I  have  dwelt  at  some  length  upon  the  local  and  temporary 
variations  in  the  profile  of  equilibrium,  in  order  to  make  clear 
their  essential  unimportance  so  far  as  the  whole  cycle  of  shore 
profile  development  is  concerned.  This  is  the  more  necessary 
because  the  true  significance  of  these  changes  has  not  been  as 
widely  understood  as  one  could  wish  were  the  case.     Long  ar- 


MATURE   STAGE  221 

tides  have  been  written,  extended  discussions  have  been  carried 
on,  and  numerous  erroneous  laws  of  shoreline  activity  have  been 
laid  down,  all  based  on  ol^servations  of  minor  fluctuations  in  the 
shore  profile  of  equilibrium.  This  has  been  unfortunate  for  the 
development  of  that  part  of  the  science  of  physiography^  relating 
to  shorelines,  for  two  reasons:  It  has  concentrated  attention  on 
the  less  important  details  of  shore  activities,  and  caused  a  neg- 
lect of  the  broader  and  more  fundamental  aspects  of  coast 
erosion;  and  it  has  led  to  endless  controversy  regarding  the 
conditions  of  wave  erosion  and  deposition,  and  the  relative  im.- 
portance  of  waves,  winds,  and  tides  in  controlling  the  direction 
of  debris  migration  along  the  coast. 

The  problem  of  longshore  debris  migration  will  be  taken  up 
in  a  later  paragraph.  Emphasis  may  here  be  laid  upon  the  fact 
that  the  shore  profile  of  equilibrium  represents  a  condition  of 
balance,  not  between  two  forces  but  between  many  forces. 
Whether  a  beach  will  be  eroded  or  will  have  material  added  to 
it  does  not  depend  upon  the  number  of  waves  which  arrive  per 
minute;  nor  upon  whether  the  waves  are  groundswells  or  local 
wind  waves;  nor  upon  whether  the  waves  strike  the  beach 
obliquely  or  at  right  angles;  nor  upon  whether  the  wind  blows 
with  the  waves  or  against  them;  nor  upon  whether  the  waves 
run  with  the  tide  or  against  it;  nor  upon  whether  the  waves 
are  of  the  oscillatory  or  translatory  variety.  Absolute  rules 
regarding  the  behavior  of  beaches  under  each  of  the  above  con- 
ditions have  been  published,  some  of  which  have  been  quoted 
on  previous  pages.  Yet  all  these  rules  are  necessarily  fallacious 
because  they  take  no  account  of  the  fundamental  fact  that 
beach  erosion  or  deposition  must  ultimately  depend  upon  whether 
or  not  the  profile  is  in  equilibrium  with  the  resultant  of  all  the 
forces  operating  upon  it.  In  a  complex  of  forces,  it  is  not  per- 
missible to  pick  out  some  one  force  and  attempt  to  build  theories 
upon  its  sole  activity;  for  it  may  well  happen  that  its  effect 
may  be  overcome  by  the  superior  power  of  other  forces  associ- 
ated with  it.  We  can  thus  readily  understand  the  fact  that  every 
"  rule  of  thumb,"  relating  to  wave  action  on  beaches,  yet  pro- 
posed has  been  vigorously  assailed  by  men  whose  observations 
directly  contradicted  it.  I  shall  hope  to  show  in  the  pages 
which  follow  that  the  matters  thus  elaborately  debated  are  of 
relatively  small  consequence,  in  view  of  the  fact  that  the  ultimate 


222  DEVELOPMENT  OF  THE  SHORE   PROFILE 

tendency  of  all  wave  action  is  to  erode  the  lands.  The  tempo- 
rary variations  in  beach  and  bench  profiles  are  insignificant 
incidents  in  the  relentless  advance  of  the  waves  into  the  heart 
of  the  continents. 

Effect  of  Longshore  Currents.  —  Thus  far  attention  has  been 
directed  to  the  very  temporary  changes  in  the  shore  profile  result- 
ing from  variations  in  the  activity  of  on-  and  offshore  forces.  Let 
us  now  consider  the  effect  of  longshore  current  action  upon  the 
shore  profile  of  maturity.  In  the  first  place,  if  the  longshore 
action  be  in  the  nature  of  beach  drifting  it  is  evident  that  any- 
thing which  locally  stops  that  movement  must  force  a  readjust- 
ment of  profiles  on  both  sides  of  the  obstruction.  For  there  will 
be  an  undue  accumulation  of  material  on  the  near  side  of  the 
obstruction,  causing  a  prograding  of  the  shore  until  the  profile  is 
steep  enough  to  allow  the  offshore  forces  to  dispose  of  the  excess 
material.  On  the  far  side  of  the  obstruction  the  shoreline  will 
be  retrograded,  because  the  failure  of  the  longshore  supply  of 
debris  will  leave  the  shore  forces  with  an  excess  of  energy  which 
will  be  expended  in  erosion.  It  is  for  this  reason  that  the  erec- 
tion of  a  pier  or  groin,  extending  out  from  a  gravelly  or  sandy 
beach,  is  usually  followed  by  an  advance  of  the  beach  on  one 
side  and  a  cutting  away  of  the  beach  on  the  other  side  of  the 
structure. 

If  the  longshore  movement  be  in  the  nature  of  a  more  exten- 
sive current  located  some  distance  offshore,  the  results  may  be 
far  more  impressive.  Imagine  a  shore  in  which  the  profile  of 
equilibrium  is  estabUshed,  and  is  being  gradually  pushed  land- 
ward under  wave  attack,  accompanied,  of  course,  by  the  minor 
fluctuations  in  beach  profiles  which  have  been  discussed  above. 
Now  let  us  suppose  that  a  broad  current  of  any  type  flows  paral- 
lel with  the  shore,  bringing  with  it  much  debris,  a  part  of  which 
is  deposited  in  the  offshore  zone.  Continued  deposition  shal- 
lows the  water,  thus  favoring  the  development  of  waves  of  trans- 
lation. As  we  have  already  seen,  waves  of  this  type  tend  to 
remove  the  deposited  material  from  the  bottom  and  drive  it 
landward,  adding  it  to  the  front  of  the  beach.  Normally,  the 
effect  of  this  action  is  to  leave  deeper  water  offshore,  which  is 
in  turn  unfavorable  for  the  development  of  waves  of  translation. 
But  in  the  case  before  us  the  longshore  current  continually 
shallows  the  bottom  by  deposition;    hence  waves  of  translation 


MATURE  STAGE  223 

may  continually  form,  and  constantly  add  material  to  the  front 
of  the  beach.  Just  so  long  as  the  current  aggrades  (builds  up) 
the  seabottom  offshore,  the  waves  will  prograde  (build  forward) 
the  shore.  Following  Davis  we  may  call  any  shore'  which  is 
experiencing  such  a  long-continued  advance  into  the  sea,  a 
prograding  shore,  and  distinguish  it  from  the  more  usual  retreat- 
ing or  retrograding  shore. 

The  prograding  of  a  shoreline  may  take  place  rapidly  or 
slowly,  and  may  continue  for  a  few  years,  a  few  centuries,  or 
many  thousands  of  years.  According  to  Marindin^^  the  beach 
at  Siasconsett,  Nantucket  Island,  has  advanced  255  meters  be- 
tween the  years  1846  and  1890.  A  beach  in  front  of  a  marine 
cliff  at  Nantasket,  Massachusetts,  has  grown  seaward  400  meters 
or  more  during  a  period  estimated  at  one  to  three  thousand 
years.^^  The  shore  of  the  Darss,  in  northern  Germany,  has  been 
prograded  7000  or  8000  meters  since  about  the  year  2000  B.  C."" 
and  a  somewhat  greater  length  of  time  was  probably  required 
for  the  advance  of  Cape  Canaveral  a  similar  distance  into  the  sea. 
It  should  be  borne  in  mind,  however,  that  these  long-continued 
additions  to  the  land,  while  far  more  important  and  significant 
than  the  minor  fluctuations  previously  discussed,  are  themselves 
only  temporary  effects  of  longer  duration,  and  that  in  a  compara- 
tively short  fraction  of  the  whole  shoreline  cycle  they  must  be 
cut  away.     This  point  will  be  further  considered  on  later  pages. 

Longshore  currents  which  have  a  fairly  high  velocity  but  which 
bring  little  or  no  sediment  to  deposit,  may  help  to  keep  the 
marine  bench  swept  clean  of  debris,  thus  materiallj'  aiding  the 
retrograding  of  the  shoreline.  It  is  probable  that  some  of  the 
localities  where  a  broad  marine  bench  is  usualh^  well  exposed, 
as  for  example  off  some  parts  of  the  coast  of  Brittanj^,  owe  the 
exposure  of  the  bench  to  the  effective  assistance  which  wind, 
tidal,  or  other  currents  lend  to  the  normal  on-  and  offshore 
processes. 

There  remain  for  consideration  one  or  two  minor  features  of 
the  shore  profile  of  maturity.  -  The  landward  portion  of  the 
profile  is  apt  to  be  complicated  by  a  series  of  ''  storm  beaches  " 
or  "  storm  terraces,"  representing  the  effects  of  waves  of  vary- 
ing dimensions  at  different  heights  of  the  tide.  Such  backshore 
terraces  often  have  a  faint  landward  slope  on  their  upper  sur- 
faces.    This  is  due  to  the  fact  that  overwash  from  the  highest 


224 


DEVELOPMENT  OF  THE  SHORE   PROFILE 


V 


waves  flows  across  the  terrace,  depositing  a  larger 
proportion  of  its  load  near  the  seaward  edge, 
where  its  velocity  is  first  checked  and  its  volume 
is  rapidly  decreasing  from  loss  of  water  sinking 
into^the  porous  beach  deposit.  The  front  of  each 
terrace  may  represent  the  upper  part  of  a  former 
beach  profile  of  equilibrium;  or  it  may  be  an 
erosion  scarp  if  waves  at  lower  levels  have  cut 
into  the  former  profile,  instead  of  merely  deposit- 
ing debris  in  front  of  and  upon  it.  Beach 
cusps^°  may  give  the  front  of  any  backshore 
terrace  a  serrate  plan. 

The  shoreface  terrace  has  an  upwardly  con- 
vex profile  at  its  seaward  margin,  as  we  have 
already  noted;  but  the  foot  of  the  terrace  may 
have  a  concave  profile  due  to  deposition  from 
suspension. -1 

Old  Stage.  —  In  Figure  37  the  profile  a^a^a^ 
represents  a  partially  submerged  land  mass  with 
a  mature  shore  profile  at  a^  where  we  see  the 
cliff,  bench,  and  terrace  which  are  shown  on 
a  larger  scale  in  Figure  35,  d^d^d^.  On  the  scale 
of  Figure  37  it  is  not  practicable  to  represent 
the  beach  deposit  on  the  bench,  but  its  presence 
may  be  inferred.  The  profile  ¥¥¥  is  the  profile 
of  early  old  age,  and  c^c^c^  the  profile  of  advanced 
old  age  of  this  same  shore.  It  will  be  noticed 
that  in  early  old  age  the  cliff  (b-)  has  a  very 
faint  slope,  scarcely  meriting  the  name  "cliff," 
except  in  a  technical  sense;  for  wave  erosion 
must  proceed  slowly  when  the  waves  have  to 
traverse  the  vast  expanse  of  shallow  water  over 
the  wide  abrasion  platform  which  they  them- 
selves have  cut,  and  when  all  debris  must  slowly 
be  moved  from  the  cliff  to  the  edge  of  the  terrace 
at  V  before  it  reaches  a  final  resting  place.  The 
marine  bench  is  still  to  be  found  in  front  of 
the  cliff;  but  it  merges  imperceptibly  into  the 
similar  but  much  larger  and  more  faintly  in- 
clined erosion  surface  which  we  have  just  referred 


OLD  STAGE  225 

to  as  the  abrasion  platform.  The  continental  terrace  (¥)  is 
developed  on  a  large  scale;  and  the  abrasion  platform,  normally 
covered  with  a  thin  marine  veneer  in  excessively  slow  transit,  is 
now  so  broad  that,  in  combination  with  the  continental  terrace, 
it  gives  a  very  extensive  continental  shelf.  One  would  scarcely 
expect  all  eroded  debris  to  be  transported  to  such  a  distance  as 
to  leave  a  continental  shelf  consisting  wholly  of  an  abrasion  plat- 
form, although  this  appears  to  be  Vogt's  idea  of  the  "  konti- 
nentale  plattform"  off  the  northern  coast  of  Norway.^-  Erosion 
and  weathering  have  reduced  the  former  upland  (a^)  to  a  series 
of  broad  valleys  separated  by  subdued  divides  of  moderate 
elevation  {¥). 

In  advanced  old  age  the  cliff  (c^)  has  been  pushed  much  farther 
inland,  and  is  so  low  and  flat  that  it  is  almost  imperceptible. 
The  remaining  land  area  has  been  worn  down  to  a  peneplane  of 
faint  relief.  Abrasion  platform  (c-)  and  continental  terrace  (d) 
are  much  broader  than  before.  Waste  is  supplied  so  slowly 
from  the  land  that  the  abrasion  platform  is  gradually  denuded 
of  its  veneer,  and  the  vast  extent  of  continental  shelf  may  con- 
sist largely  of  bare  rock  on  the  landward  side  and  sedimentary 
deposits  on  the  deep-water  side. 

Wave  Base.  —  The  final  stage  of  marine  erosion  will  have  been 
reached  when  the  entire  land  mass  is  reduced  to  an  ultimate  abra- 
sion platform  surrounded  on  all  sides  by  a  continental  terrace,  the 
level  of  the  platform  being  as  far  below  the  surface  of  the  sea 
as  wave  erosion  is  effective.  In  other  words,  the  cycle  of 
marine  denudation  is  completed  when  all  the  land  is  reduced  to 
the  baselevel  of  wave-erosion,  just  as  the  cycle  of  fluvial  denu- 
dation is  completed  when  all  the  land  is  reduced  to  the  baselevel 
of  stream  erosion.  The  valuable  term  "  wave  base  "  was  in- 
troduced by  Gulliver-^  to  denote  the  imaginary  plane  down  to 
which  wave  action  tends  continually  to  reduce  the  lands;  and 
since,  as  we  have  seen,  the  lower  limit  of  effective  wave  work  is 
probably  reached  at  a  depth  of  about  600  feet,  we  may  tenta- 
tively consider  wave  base  as  an  imaginary  plane  about  600  feet 
below  the  surface  of  the  sea.  A  cycle  of  wave  erosion  ends, 
therefore,  when  all  the  land  is  reduced  to  a  plane  surface  about 
600  feet  below  scale vel. 

A  common  error  is  to  confuse  wave  base  with  profile  of  equi- 
librium.    The  gently  sloping  subaqueous  terrace  bordering  lake 


226  DEVELOPMENT  OF  THE  SHORE  PROFILE 

shores  or  the  shores  of  a  sea,  and  the  submarine  platforms  of 
islands  truncated  by  ocean  waves,  are  frequently  explained  as 
the  products  of  wave  erosion  down  to  wave  base,  when  in  fact 
they  merely  represent  surfaces  of  equilibrium  which  are  very 
slowly  being  reduced  toward  a  wave  base  far  below.  The  Platte 
River  has  established  its  profile  of  equilibrium  at  a  level  which 
is  in  places  some  thousands  of  feet  above  the  sealevel,  and  to 
ordinary  observation  does  not  now  appear  to  be  cutting  its  valley 
any  deeper.  Yet  no  one  would  make  the  mistake  of  saying  that 
the  valley  floor  of  this  stream  had  been  reduced  to  baselevel. 
It  is  no  less  erroneous  to  say  that  a  subaqueous  terrace  on  which 
the  marine  forces  are  now  in  equilibrium  and  which  shows  no 
evident  indications  of  being  cut  deeper,  has  been  reduced  to 
wave  base.  The  error  is  compounded  when  the  false  assumption 
that  the  terrace  represents  wave  base  is  made  the  ground  for  the 
conclusion  that  wave  action  is  not  effective  below  a  compara- 
tively shallow  depth.  Equilibrium  miay  be  established  at  a  shal- 
low depth;  from  that  level  downward  wave  erosion  proceeds 
more  and  more  slowly,  but  none  the  less  surely. 

It  might  seem  on  first  thought  that  no  limit  could  be  set  to  the 
depth  of  wave  action,  because  theoretically  waves  of  translation 
affect  the  water  on  the  bottom  as  much  as  they  do  the  surface 
layers,  no  matter  what  the  water  depth  may  be.  We  have 
already  seen,  however,  that  waves  of  translation  are  more  apt  to 
be  formed  in  the  shallow  waters  surrounding  the  lands,  since  con- 
ditions favorable  to  their  development  seldom  exist  in  the  deep 
sea.  Another  point  of  much  importance  in  this  connection  is 
that  waves  of  translation,  when  propagated  into  deep  water,  tend 
to  change  into  oscillatory  waves,  as  has  been  shown  by  Rankine.^^ 
It  would  seem  to  follow  from  this  that  the  ordinary  waves  of 
translation  found  near  the  shores  cannot  be  efficient  agents  in 
lowering  the  level  of  wave  base,  because  they  cease  to  exist  as 
such  when  the  water  attains  anj^  considerable  depth. 

Gulliver-^  states  that  the  abrasion  platform  "  will  not  lie -as 
far  below  the  surface  of  the  sea  [in  late  old  age  of  shore  develop- 
ment] as  it  did  in  its  maturity."  Such  a  statement  suggests  that 
Gulliver  confused  the  plane  of  denudation  or  abrasion  platform 
with  the  submarine  plain  of  deposition  formed  by  the  marine 
veneer  laid  down  upon  the  platform.  Even  so,  it  is  difficult  to 
see  how  the   marine  veneer  could  become  thicker  in  the  late  old 


OLD  STAGE 


227 


cM:::»^ 


228  DEVELOPMENT  OF  THE  SHORE   PROFILE 

age  of  shore  development,  thereby  shallowing  the  sea.  There  is 
a  slight  tendency  in  this  direction  at  an  earlier  stage;  but  in  late 
old  age  the  supply  of  debris  from  the  land  is  decreased  and  the 
abrasion  platform  must  be  denuded  of  its  veneer,  as  already 
shown. 

Validity  of  the  Theory  of  a  Marine  Cycle.  —  Thus  far  I  have 
for  the  most  part  assumed  the  theoretical  possibility  of  extensive 
marine  erosion  to  wave  base,  and  have  only  incidentally  re- 
ferred to  contrary  opinions.  It  is  only  proper,  however,  that  we 
fairly  consider  any  objections  to  the  theory  of  marine  planation 
and  determine  whether  they  invalidate  any  of  the  conclusions 
reached  above. 

Wave-cut  Benches.  —  One  finds  no  reason  to  doubt  that  wave 
erosion  has  produced  more  or  less  plane  surfaces  of  moderate 
breadth  around  the  margins  of  certain  lands.  The  very  striking 
pre-glacial  shore  terrace  (Plate  XXVI)  bordering  the  western 
isles  of  Scotland  is  described  by  Wright^^  as  an  uplifted  platform 
of  marine  erosion  having  a  breadth  of  about  half  a  mile  in  places, 
part  of  the  breadth  having  been  lost  through  later  wave  erosion 
at  present  sealevel.  Lawson-'^  has  described  uplifted  wave-cut 
rock  platforms  on  the  coast  of  California  having  a  maximum 
width  of  more  than  a  mile.  Comparatively  rapid  emergence  of 
the  land  prevented  long-continued  wave  attack  at  one  horizon, 
with  the  result  that  the  platforms  constitute  an  extensive  series 
of  berraces  (Fig.  38),  the  highest  of  which  is  over  1500  feet  above 
sealevel.  There  can  be  no  doubt  that  had  all  this  erosive  work 
been  performed  at  one  horizon  the  resulting  platform  would 
have  been  much  broader  than  any  one  of  the  existing  wave-cut 
surfaces.  Comparatively  weak  waves  on  Lake  Michigan  attack- 
ing shores  of  glacial  drift  have  formed  a  terrace  whose  outer 
margin  is  approximately  60  feet  below  the  lake  surface,  and 
which  varies  in  breadth  from  2  to  6  miles,  with  a  maximum  at  one 
locality  of  12  miles.  Andrews^^  assumed  that  the  entire  breadth 
of  the  terrace  was  due  to  wave  erosion ;  and  proceeding  on  the 
further  assumption  that  the  rate  of  wave  erosion  is  the  same 
during  all  stages  of  terrace  cutting,  he  used  the  breadth  of  the  ter- 
race and  the  present  known  rate  of  cliff  retreat  to  establish  a 
measurement  of  post-glacial  time.  Both  his  assumptions  must 
be  considered  erroneous;  but  it  seems  probable  that  from  one  to 
several  miles  of  the  terrace  breadth  is  wave-cut,  even  though  a 


(229) 


230  DEVELOPMENT  OF  THE  SHORE  PROFILE 

large  part  of  it  represents  the  effect  of  wave  deposition.  In  a 
paper  entitled  "  Fenomeni  di  abrasione  sulle  coste  dei  paesi  dell' 
Atlante  "-^  Fischer  describes  a  submarine  terrace  bordering  parts 
of  the  north  coast  of  Africa  having  an  outer  margin  approxi- 
mately 100  to  200  meters  below  the  surface  of  the  Mediter- 
ranean, and  a  maximum  breadth  of  at  least  12  miles.  The  en- 
tire breadth  of  this  terrace  is  regarded  bj^  Fischer  as  a  marine 
abrasion  platform;  but  it  seems  probable  that  the  outer  part 
of  it  is  of  constructional  origin.  Good  photographic  illustrations 
of  its  exposed  landward  margin,  where  it  is  an  undoubted  plat- 
form of  marine  abrasion,  accompany  the  same  author's  report 
on  ''  Kiistenstudien  und  Reiseeindrucke  aus  Algerien,"3o  while 
a  short  description  of  the  terrace  occurs  in  an  earlier  paper  on 
"  Kiistenstudien  aus  Nordafrika."^^  It  is  well  known  that  cer- 
tain volcanoes  formed  in  the  ocean  have  been  reduced  by  wave 
erosion  to  submarine  platforms  within  the  space  of  a  few  years,32 
and  there  are  excellent  reasons  for  believing  that  many  of  the 
more  or  less  circular  submarine  platforms  in  the  Pacific  Ocean 
described  by  Wharton^^  and  other  writers,  and  more  recently 
discussed  by  Daly^^  in  connection  with  the  glacial-control  theory 
of  coral  reefs,  represent  volcanoes  whose  summits  have  been 
truncated  by  marine  abrasion.  Not  a  few  of  these  platforms 
measure  from  20  to  30  miles  or  more  in  diameter,  but  what 
portion  of  the  whole  represents  marginal  deposits  of  debris  eroded 
from  the  center  is  unknown. 

The  great  wave-cut  platform  ("  strandfladen  "  of  the  Nor- 
wegians) fringing  the  west  coast  of  Norway,  best  known  through 
the  studies  of  Reusch,^^  Richter,^^  Vogt^^  and  Nansen,^^  has  an 
average  breadth  of  nearly  30  miles,  and  a  maximum  breadth  of 
nearly  40  miles  according  to  Vogt  and  Nansen,  if  we  include  the 
portion  still  submerged.  Notwithstanding  the  doubt  implied 
by  Reusch,  and  clearly  expressed  by  Hansen^^  and  Nussbaum^o 
regarding  the  essential  marine  origin  of  this  topographic  feature 
it  is  generally  considered,  and  probably  correctly  so,  one  of  the 
best  examples  of  marine  abrasion  on  a  large  scale  yet  discovered 
along  our  present  coasts.  Nansen'*^  describes  similar  platforms 
of  marine  abrasion  fringing  the  coasts  of  Siberia,  Greenland 
and  other  land  areas,  none  of  which  are  so  broad  as  the  Norwegian 
case,  although  a  breadth  of  nearly  20  miles  is  not  unloiown. 
The  east   coast   of  India,  as   described  by  Cushing,42  consists 


VALIDITY  OF  THE  THEORY  OF  A  MARINE  CYCLE     231 

in  part  of  a  remarkably  smooth,  uplifted  plane  of  marine 
denudation,  above  which  rise  numerous  unconsumed  rem- 
nants of  quartzite,  the  bases  of  these  former  islands  or  stacks 
not  infrequently  being  marked  by  sea-caves  (Plates  XXIX- 
XXXI).  In  places  this  wave-cut  plane  attains  a  breadth  of 
about  50  miles.^^ 

It  seems  highly  probable  that  considerable  portions  of  the 
continental  shelves  bordering  certain  shores  represent  plat- 
forms of  marine  abrasion.  Nansen"'*  is  of  the  opinion  that  a 
great  part  of  the  continental  shelf  west  of  Norway  is  of  this 
origin,  and  believes  that  between  latitude  65°  10'  N.  and  66°  N. 
solid  rock  is  present  clear  to  the  edge  of  the  shelf.  Figure  39 
represents  two  of  Nansen's  sections  for  this  region,  in  which  the 
results  of  soundings  are  indicated.  Notwithstanding  the  diffi- 
culty of  determining  the  presence  of  sohd  rock  by  the  sounding 
method,  Nansen  believes  that  the  rocky  ridge  shown  near  the 
outer  margin  of  the  shelf  is  correctly  indicated.  If  he  is  right 
we  have  here  a  plane  of  marine  abrasion,  including  the  rocky 
"  coast  platform  "  described  above,  exceeding  170  miles  in 
maximmn  breadth. 

It  is  true  that  Nansen  doubts  the  power  of  waves  to  carve  a 
broad  and  gently  sloping  platform  on  a  simple  coast;  and  he 
therefore  assumes  that  even  in  the  case  of  the  narrower  "  coast 
platform  "  or  "  strandfladen  "  the  coast  was  first  deeply  in- 
dented by  fjords,  and  the  platform  later  cut  during  glacial  and 
interglacial  periods  by  wave  attack  from  both  the  ocean  and 
the  fjord  waters,  aided  by  subaerial  denudation.^^  We  must 
doubt  the  validity  of  the  theoretical  grounds  on  which  he  thus 
limits  the  power  of  waves  in  the  open  ocean;  and  must  like- 
wise doubt  whether  small  waves  in  sheltered  fjords,  formed  as 
they  are  on  the  surface  of  deep  water  and  therefore  unarmed 
with  debris,  and  subject  to  reflection  from  nearly  vertical  rock 
walls  without  opportunity  for  erosion,  could  materially  aid  in 
the  process  of  reducing  a  land  mass  to  a  submarine  platform. 
Neither  do  his  arguments  in  favor  of  the  glacial  age  of  the  coast 
platform  appear  convincing'*^  But  the  facts  presented  by  this 
author  leave  no  room  to  doul^t  the  existence  along  the  west 
coast  of  Norway  of  a  wave-carved  platform  which  is  certainly 
50  to  75  miles  broad,  and  possibly  as  much  as  170  miles  broad 
in  some  of  its  parts. 


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Page  232 


VALIDITY  OF  THE  THEORY  OF  A   MARINE  CYCLE      233 


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234  DEVELOPMENT  OF  THE  SHORE   PROFILE 

Theory  of  Marine  Abrasion.  —  It  is  evident  from  the  brief 
survey  given  above  that  planes  of  marine  abrasion  a  great  many- 
miles  in  breadth  are  well-attested  features  of  the  earth's  surface. 
But  it  is  difficult  or  impossible  to  tell  whether  or  not  these  planes 
were  formed  during  a  stiU-stand  of  the  land  or  during  a  progres- 
sive submergence.  There  is  a  widespread  idea  that  waves  can  cut 
into  a  still-standing  land  mass  only  to  a  very  moderate  extent 
before  they  will  exhaust  themselves  on  the  shallow  bench  which 
they  have  carved.  According  to  this  interpretation,  a  marine 
cliff  will  be  pushed  inland  by  the  waves  for  a  short  distance,  and 
will  then  remain  unchanged  in  position  unless  subsidence  of  the 
land  mass  deepens  the  water  on  the  marine  bench  and  thus  per- 
mits waves  once  more  to  erode  the  base  of  the  cliff.  Marine 
planation  would  only  be  possible,  therefore,  on  a  subsiding  land 
area.  This  view  is  expressed  by  von  Richthofen''^  in  his  great 
work  on  China,  where  he  states  that  slow  depression  alone  can 
produce  regional  abrasion,  since  without  progressive  sinking  the 
waves  soon  become  exhausted  on  a  narrow  platform  of  their 
own  carving.  The  same  idea  is  expressed  in  his  ''Ftihrer  fiir 
Forschungsreisende  "''^  De  Martonne,  in  his  "  Traite  de  Geo- 
graphie  Physique  "^^j  de  Lapparent  in  his  "  Traite  de  Geologic  "^ 
and  his  "  Legons  de  Geographie  Physique  "^^;  Kayser  in  his 
"  Lehrbuch  der  Geologie  "^^;  and  Scott  in  his  "Introduction  to 
Geology  "^^  are  among  the  text-book  writers  who  have  adopted 
von  Richthofen's  theory  that  waves  cannot  cut  far  into  the  land 
unless  wave  erosion  is  aided  by  coastal  subsidence.  Many  others 
have  taken  the  same  position,  and  some  have  even  gone  so  far  as 
to  cite  w^ave  erosion  as  an  indication  of  land  sinking.  For  exam- 
ple, Hahn^^  says  we  must  suspect  a  sinking  of  every  region  which 
suffers  loss  through  the  washing  away  of  its  margin,  and  Haage^^ 
gives  wave  erosion  as  one  of  the  distinguishing  characteristics  of 
a  sinking  coast. 

On  the  other  hand,  there  are  a  few  who  have  maintained  that 
waves  will  continue  to  cut  into  any  land  mass  so  long  as  it  pro- 
jects above  sealevel,  whether  or  not  it  is  undergoing  depression. 
Ramsay,  the  first  to  recognize  the  power  of  the  waves  to  produce 
a  plane  of  abrasion,  clearly  expresses  his  belief  that  while  sub- 
sidence of  a  land  mass  will  aid  the  process  of  marine  erosion,  it 
is  not  essential;  since,  "  taking  unlimited  time  into  account," 
any  land  area  must  eventually  be  worn  away  by  the  waves^^. 


VALIDITY  OF  THE  THEORY  OF  A   MARINE  CYCLE      235 

Green  seems  equally  convinced  of  the  ability  of  wave  erosion  to 
produce  an  extensive  plane  of  denudation  without  subsidence,  as 
he  explains  the  origin  of  such  planes  without  mentioning  changes 
of  level".  Both  of  these  authors  failed  to  appreciate  the  con- 
siderable depths  to  which  wave  action  extends,  Ramsay  assuming 
that  "  the  line  of  denudation  "  is  "a  level  corresponding  to  the 
average  height  of  the  sea,"  while  according  to  Green  marine 
denudation  must  reduce  a  country  to  ''an  even  surface  coin- 
ciding approximately  with  the  level  of  the  lowest  tides."  Jukes- 
Browne^^  was  almost  as  conservative  in  his  estimate  of  the 
depth  of  marine  erosion.  Davis^^,  Gulliver*^",  and  Fenneman^^ 
are  among  those  who  recognize  not  only  the  possibility  of  in- 
definite wave  erosion  on  a  stable  land  mass,  but  the  additional 
fact  that  the  baselevel  of  wave  erosion  is  located  at  an  appreci- 
able depth  below  the  water  surface. 

In  the  opinion  of  the  writer  any  careful  analysis  of  the  process 
of  marine  erosion  must  lead  to  the  conclusion  that  marine  pla- 
nation  is  possible  without  coastal  subsidence.  We  have  already 
seen  that  where  the  resultant  of  wave  action  is  landward,  material 
is  driven  toward  the  shore  until  the  steepening  of  the  shore  pro- 
file produces  a  condition  of  equilibrium  in  which  material  driven 
up  the  slope  by  the  landward-acting  forces  returns  again  under 
the  combined  influence  of  gravity,  undertow,  and  other  seaward- 
acting  forces.  During  the  entire  period  of  equilibrium,  sand-, 
pebbles,  and  shingle  are  driven  back  and  forth,  up  and  down  the 
beach  slope,  continually  grinding  themselves  finer  and  finer. 
In  this  gigantic  mill  which  borders  the  lands,  rock  fragments 
are  continually  reduced  to  a  state  of  such  exceedingly  fine  com- 
minution that  they  are  readily  removed  from  the  shore'and  shore- 
face  zones  as  suspended  particles  in  the  water.  During  and 
after  heavy  wave  action  the  water  is  turbid  with  matter  in 
suspension  to  a  considerable  distance  from  the  land.  Part  of 
this  suspended  matter  is  removed  far  from  shore  by  the  many 
currents  which  are  involved  in  oceanic  circulation,  and  finds  a 
permanent  resting  place  beneath  the  quiet  waters  of  abysmal 
depths. 

If  there  is  an  absolute  loss  of  land  where  the  resultant  of  wave 
action  is  landward,  and  the  shore  profile  is  built  forward  until 
equilibrium  is  established,  how  much  greater  must  be  that  loss 
when  the  seaward  components  of  wave  agitation  prevail  and 


236 


DEVELOPMENT  OF  THE  SHORE  PROFILE 


X 
X 


VALIDITY   OF  THE  THEORY  OF  A   MARINE  CYCLE      237 

coarser  debris  on  the  bottom,  as  well  as  material  in  suspension, 
is  transported  seaward  to  be  deposited  over  the  edge  of  the 
continental  shelf.  Account  must  also  be  taken  of  the  fact  that 
agitation  of  the  marine  veneer  is  continually  grinding  its  par- 
ticles smaller  and  grinding  material  from  the  solid  surface  of 
the  abrasion  platform,  thus  producing  fine  sediment  which  cur- 
rents may  readily  carry  to  deeper  water  during  and  after  vigorous 
wave  action.  The  never-ending  shifting  of  the  beach  deposit 
back  and  forth  over  the  shore  and  shoreface,  already  fully  de- 
scribed, is  accompanied  by  a  ceaseless  loss  of  the  finest  attrition 
products.  Whether  the  shore  profile  is  in  equilibrium  or  not, 
whether  waves  are  depositing  beach  material  or  sapping  cliff 
bases,  whether  the  marine  veneer  is  increasing  or  decreasing  in 
volume,  there  is  a  constant  loss  of  very  fine  matter  which  is 
borne  far  away  to  deep  water  by  current  action.  This  means 
an  eventual  loss  of  equilibrium  which  must  ultimately  be  re- 
stored by  the  erosion  of  more  material  from  the  lands.  Where- 
ever  there  is  wave  erosion  there  is  an  absolute  loss  of  material 
from  the  lands  attacked.  The  laws  of  wave  action  afford  no 
basis  for  Mitchell's  conclusion  that  the  sea  restores  to  the  con- 
tinent "  all  the  material  washed  from  its  bluffs  and  headlands  "^^. 
On  the  contrary,  we  must  conclude  that  the  sea  never  restores 
anything  to  the  continent,  except  temporarily.  The  surface 
extent  of  lands  temporarily  built  by  marine  agencies  may  be 
great,  but  their  total  volume  above  sealevel  is  small  as  compared 
with  the  volume  removed  by  marine  erosion.  There  was  much 
of  truth  in  the  statement  made  nearly  a  century  ago  by  Robert 
Stevenson,  to  the  effect  that  "  these  apparent  acquisitions  are 
no  more  to  be  compared  with  the  waste  alluded  to,  than  the 
drop  IS  to  the  water  of  the  bucket  ^^".  In  the  end  the  tempo- 
rarily restored  materials  must  themselves  suffer  removal  by  the 
combined  action  of  waves  and  currents,  which,  however  slowly, 
yet  unceasingly  destroy  any  land  mass  exposed  to  their  attack. 
Under  the  most  unfavorable  conditions  the  loss  from  the  lands 
will  be  small,  but  real.  Where  conditions  favor  vigorous  wave 
erosion,  rapid  disintegration  of  the  rock  fragments,  extensive 
solution  of  the  rock-forming  minerals,  efficient  transportation 
of  the  mechanical  debris  offshore,  and  high  current  velocities 
continued  to  deep  water,  the  wasting  of  the  land  may  be  ex- 
ceedingly rapid. 


238  DEVELOPMENT  OF  THE   SHORE   PROFILE 

It  is  not  possible  that  waves  should  exhaust  themselves  upon 
a  platform  of  their  own  carving,  and  thus  fail  after  a  time  to  con- 
tinue cliff  erosion;  for  the  loss  of  wave  energy  means  that  that 
energy  has  been  expended  upon  the  platform  in  question,  and 
energy  so  expended  can  have  but  one  result:  abrasion  and  con- 
sequent lowering  of  the  platform.  This  partially  removes  the 
cause  of  wave  exhaustion,  so  that  later  waves  reach  the  cliff 
base  with  enough  energy  remaining  to  effect  some  slight  erosion. 
Any  surface  shallow  enough  to  retard  wave  attack  must  suffer 
denudation  until  the  attack  is  resumed.  The  vertical  limit  of 
marine  denudation  is  a  surface  so  low  that  wave  action  is  no 
longer  retarded  by  it.  The  corollary  of  this  is  that  there  is  no 
horizontal  limit  of  marine  erosion. 

In  this  connection  it  should  be  pointed  out  that  fairly  rapid 
wave  cutting  may  occur  at  the  base  of  a  cliff  which  has  been 
pushed  far  into  the  land.  This  arises  from  the  fact  that  the 
shore  profile  must  change  extensively  at  times  because  of  large 
variations  in  the  forces  attacking  the  shore.  Imagine,  for  ex- 
ample, that  on  a  shore  which  had  been  in  nearly  perfect  equi- 
librium under  gradually  weakening  wave  attack  for  so  long  a 
time  that  the  beach  deposit  had  wasted  away  to  a  very  small 
volume,  a  series  of  unusual  storms  should  drive  in  vigorous 
oscillatory  waves  and  develop  a  strong  undertow.  It  is  quite 
conceivable  that  the  cliff,  which  had  for  years  scarcely  been 
touched  by  the  waves,  might  be  steepened  and  driven  inland 
with  comparative  rapidity.  In  this  case  the  average  rate  of  cliff 
retreat  would  be  exceedingly  small;  but  the  absolute  rate  for  a 
hmited  time  might  be  high.  Variations  in  the  direction  and 
strength  of  longshore  currents  might  also  be  accompanied  by 
increased  rate  of  cliff  recession,  in  any  place  where  such  vari- 
ations materially  affected  the  condition  of  the  shore  profile. 
Even  fairly  rapid  cliff  retreat  on  a  late  mature  or  old  shore 
profile  is  not,  therefore,  necessarily  proof  that  coastal  sub- 
sidence has  admitted  laj'ger  waves  by  deepening  the  water 
offshore. 

Effect  of  Deposition.  —  One  might  suppose  that  the  deposition 
of  organic  or  chemical  sediments  from  the  water  above  the  abra- 
sion platform  would  protect  the  platform  from  erosion  and  by 
preventing  its  further  deepening  eventually  stop  further  cliff  ero- 
sion.    But  a  little  consideration  will  show  that  such  deposition 


VALIDITY  OF  THE  THEORY  OF  A   MARINE   CYCLE      239 


240  DEVELOPMENT  OF  THE   SHORE   PROFILE 

can  only  bring  about  an  elevation  of  the  general  level  at  which 
marine  planation  will  occur.  In  the  early  part  of  the  shore  cycle, 
when  deposition  is  small  and  erosion  vigorous,  the  platform  will 
be  rapidly  lowered.  Later,  when  the  increased  depth  of  water 
over  the  platform  permits  more  extensive  deposition  of  organic 
or  chemical  sediment  from  the  increased  volume  of  superjacent 
water,  and  at  the  same  time  results  in  decreased  intensity  of 
wave  erosion  on  the  platform,  the  contending  forces  will  be 
more  nearly  balanced.  Equilibrium  will  be  established  and  the 
effective  wave  base  reached,  when  wave  agitation  and  current 
action  combined  can  just  effect  the  removal  of  the  deposits 
which  tend  to  accumulate  on  the  platform.  Were  it  not  for  the 
burden  of  removing  these  deposits,  the  waves  would  reduce  the 
platform  still  lower. 

It  is  sometimes  stated  that  before  waves  can  erode  cliffs  far 
into  the  lands,  rivers  will  bring  out  vast  quantities  of  sediment 
which  will  in  turn  be  widely  distributed  along  the  shores  by 
currents.  Deposition  of  the  sediment,  it  is  argued,  will  shallow 
the  water,  protect  the  shores  and  sea-bottom,  and  effectively  pre- 
vent further  cliff  retreat.  This  argument  assumes  that  the 
quantity  of  debris  brought  out  by  rivers  and  distributed  along 
the  margins  of  the  lands  is  equal  to  or  greater  than  the  quantity 
of  debris  which  the  marine  forces  are  competent  to  remove, 
and  that  therefore  the  entire  energy  of  those  forces  is  consumed 
in  handling  river-brought  material.  While  it  seems  to  the 
writer  that  as  a  general  proposition  this  assumption  is  untenable, 
we  may  temporarily  grant  its  reasonableness  for  sake  of  argument, 
providing  it  refers  to  a  youthfully  or  maturely  dissected  land 
mass.  As  the  land  wears  lower  and  the  streams  become  more 
sluggish,  the  latter  will  bring  to  the  sea  a  decreasing  amount  of 
sediment,  an  increasing  proportion  of  which  will  be  carried  in 
suspension  and  so  will  be  borne  out  to  deep  water  without 
pausing  in  the  vicinity  of  the  shores.  The  forces  of  marine 
erosion  and  transportation  will  eventually  remove  the  deposits 
which  impeded  wave  attack  during  an  earlier  part  of  the  fluvial 
cycle  of  land  dissection,  and  once  more  the  relentless  encroach- 
ment of  the  sea  will  be  manifest.  Under  the  assumption  least 
favorable  to  wave  erosion,  therefore,  the  progress  of  marine 
planation  cannot  be  stopped  by  river-brought  sediment.  It 
can  only  be  delayed.     Deltas  may  be  built  seaward  against  the 


VALIDITY  OF   THE   THEORY   OF  A   MARINE   CYCLE      241 


Plate  XXX. 


Photo  by  S.  TF.  Ciishing. 
Base  of  monadnock  iii  Plate  XXIX,  showing  effects  of  marine  erosion. 


242  DEVELOPMENT   OF  THE  SHORE   PROFILE 

weves  for  a  time,  and  help  to  keep  parts  of  the  shoreline  young; 
late-mature  coasts  are  delta-free. 

The  direct  effects  of  river  sediments  in  preventing  cliff  erosion 
have  been  exaggerated,  as  intimated  in  the  foregoing  paragraph. 
Near  the  mouth  of  many  rivers  it  is  perfectly  apparent  that  the 
river  deposits  are  directly  shielding  the  cliffs  from  wave  attack. 
But  active  clifRng  is  going  on  along  many  other  coasts  in  spite 
of  the  fact  that  numerous  streams  enter  the  sea  through  valleys 
opening  in  the  face  of  the  cliffs;  while  long  stretches  of  coast 
have  enormous  accumulations  of  beach  deposits  demonstrably  not 
of  fluvial  origin.  When  we  come  to  consider  the  deposits  in  the 
offshore  zone,  however,  it  is  probable  that  greater  importance 
must  attach  to  stream-brought  sediments,  and  that  indirectly 
they  may  play  an  important  role  in  certain  stages  of  the  marine 
cycle.  Let  us  analyze,  if  possible,  the  relation  of  the  marine  cycle 
to  the  fluvial  cycle  of  land  dissection. 

Correlation  of  the  Marine  and  Fluvial  Cycles.  —  Imagine  a 
newly  uplifted  land  mass  of  great  areal  extent  and  irregular  sur- 
face, attacked  at  once  by  wave  and  stream  erosion.  During  the 
youthful  stage  of  stream  development  sediment  is  being  eroded 
from  some  parts  of  the  stream  profile  only  to  be  deposited  else- 
where as  filling  for  lake  basins,  as  alluvial  fans,  flood  plains,  and 
other  temporary  accumulations.  Only  a  small  part  of  the  sedi- 
ment reaches  the  sea.  During  this  period  the  shore  profile  is 
in  the  young  and  perhaps  early  mature  stages  of  its  development, 
the  abrasion  platform  is  being  rapidly  developed,  and  the  cliffs 
are  being  pushed  steadily  inland.  Since  river-brought  sediment 
is  small  in  amount  at  this  time,  the  development  of  the  shore 
profile  is  not  greatly  affected  by  it. 

When  the  drainage  system  on  the  land  is  thoroughly  inte- 
grated, all  its  parts  nicely  adjusted,  and  the  stream  profiles  of 
equilibrium  perfected,  sediment  from  all  parts  of  the  land  sur- 
face is  ceaselessly  swept  seaward.  At  the  river  mouths  the 
coarser  sediment  may  be  deposited  in  a  delta,  or  driven  along  the 
shores  in  either  direction.  The  finer  material  will  be  trans- 
ported to  a  greater  or  less  distance  by  some  of  the  many  types  of 
marine  currents,  and  much  of  it  deposited  in  the  offshore  zone. 
B}^  this  time  the  marine  cliff  has  been  pushed  well  into  the  land, 
the  abrasion  platform  has  attained  a  considerable  width,  and  a 
thin  layer  of  marine  veneer  is  journeying  slowly  down  the  sub- 


CORRELATION  OF  THE  MARINE  AND  FLUVIAL  CYCLES      243 

marine  slope  toward  the  edge  of  the  continental  terrace.  In- 
creasing volumes  of  river-brought  sediment  are  now  deposited 
upon  the  shelf,  adding  to  the  thickness  of  the  marine  veneer  and 
continental  terrace,  and  thereby  shallowing  the  water  of  the  off- 
shore zone.  Aggrading  will  continue  until  a  level  is  reached  where 
the  increased  wave  and  current  agitation  is  sufficient  to  remove 
the  amount  of  debris  which  is  deposited.  The  new  profile 
of  equilibrium  will  hardly  rise  to  the  surface  of  the  sea,  except 
under  exceptional  conditions  in  limited  areas,  as  where  deltas 
are  temporarily  formed.  Coasts  bordering  shallow  inland  seas, 
or  otherwise  protected  from  the  full  attack  of  destructive  marine 
forces,  may,  like  the  coasts  of  Holland  and  Belgium,  be  built  far 
forward  by  delta  accumulations  before  the  inevitable  period  of 
their  removal  begins.  Especially  will  this  be  the  case  if  lands 
raised  to  mountainous  heights  shed  debris  into  the  sea  through 
many  rivers  with  exceptional  rapidity.  Both  modern  and 
ancient  examples  of  coasts  where  conditions  favored  extensive 
delta  growth  are  cited  by  Barrel^*,  who  fully  recognized  the  tem- 
porary nature  of  the  delta  protection  of  coasts.  Around  most 
of  the  land  margin,  where  delta  protection  is  lacking,  smaller 
waves  will  continue  to  traverse  the  waters  shallowed  by  the 
deposition  of  river-brought  debris,  and  will  continue  to  erode 
the  cliff,  but  more  feebly  than  before.  Cliff  retreat,  already 
slow  because  of  the  increasing  breadth  of  the  offshore  zone,  will 
be  still  further  retarded  by  virtue  of  the  decreased  depth  of 
water  in  that  zone.  Maturity  of  land  drainage,  therefore, 
means  retarded  shoreline  development. 

As  the  rivers  of  the  land  approach  old  age,  they  become  more 
sluggish.  Meandering  in  circuitous  courses  on  a  very  low 
gradient,  they  can  transport  but  a  limited  volume  of  the  finest 
sediment.  Decreased  land  relief  is  accompanied  by  decreased 
rainfall  and  increased  loss  of  water  by  evaporation;  and  this 
means  diminished  stream  volume.  A  smaller  quantity  of  finer 
debris  is  weathered  from  the  very  gentle  slopes  of  the  old  valley 
sides,  to  be  carried  to  the  sea  by  shrunken  and  enfeebled  rivers. 
More  material  is  removed  in  solution;  less  and  finer  material  is 
removed  in  mechanical  suspension.  Upon  reaching  the  sea  much 
or  all  of  this  very  fine  material  may  be  transported  far  from  the 
land  by  marine  currents  before  deposition  is  possible.  Waves 
and  currents  in  the  offshore  zone  are  no  longer  over-burdened 


244  DEVELOPMENT  OF  THE  SHORE  PROFILE 

with  river  deposits,  and  expend  their  excess  energy  in  removing 
the  material  previously  deposited.  The  depth  of  the  water  is 
increased  until  the  abrasion  platform  is  again  exposed  to  the 
slow  wear  of  the  migrating  marine  veneer.  Larger  waves  gain 
access  to  the  land,  and  cliff  erosion  is  relatively  more  effective 
than  before.  But  increased  breadth  of  the  abrasion  platform 
and  continental  terrace  compels  material  eroded  from  the  cliffs 
to  make  a  longer  journey  to  deep  water;  and  the  longer  time 
necessary  to  dispose  of  cliff  debris  necessarily  tends  to  retard 
cliff  recession.  On  the  other  hand,  the  reduction  in  land  relief 
accomplished  by  the  subaerial  forces  gives  a  lower  marine  cliff 
from  which  a  smaller  amount  of  debris  is  offered  to  the  waves 
and  currents  for  removal.  Current  action  along  the  smoothed- 
out  contours  of  mature  and  old  shores  may  be  much  more  effec- 
tive than  on  the  more  irregular  shores  of  youth;  and  the  conse- 
quent more  effective  removal  of  debris  from  the  shores  may  com- 
pensate, in  part  at  least,  for  the  greater  distance  to  which  it  must 
be  removed.  All  things  considered,  it  seems  to  the  writer  that 
the  retrograding  of  the  shoreline  must  proceed  more  rapidly 
during  the  old  age  of  land  dissection  than  during  its  maturity. 
Especially  must  this  be  true  where  the  reduction  of  extensive 
land  areas  by  subaerial  denudation  causes  a  progressive  rise  of 
sealevel  due  to  the  infilling  of  sediments  in  the  ocean  basins. 
The  significance  of  these  relationships  in  the  cycle  of  marine 
sedimentation  has  already  been  ably  discussed  by  Barrell'^^  in 
his  essay  on  the  ''  Relative  Geological  Importance  of  Continental, 
Littoral,  and  Marine  Sedimentation." 

The  foregoing  considerations  lead  to  the  interesting  conclusion 
that,  other  things  being  equal,  marine  erosion  should  proceed 
most  effectively  about  the  low-lying  desert  areas  of  tropical 
regions,  especially  on  the  windward  sides  of  such  areas.  For 
the  absence  of  rivers  would  permit  the  development  of  the  shore 
cycle,  unretarded  by  any  land  sediments  except  the  very  fine 
material  borne  seaward  by  the  winds.  On  the  windward  side 
even  the  seolian  deposits  would  be  lacking,  while  the  onshore 
winds  would  continually  drive  vigorous  waves  against  the  cliffs, 
and  by  elevating  the  water  surface  would  tend  to  produce  a  strong 
undertow  which  would  assist  in  removing  the  products  of  wave 
erosion  to  deep  water.  On  the  leeward  side  the  interfering  ac- 
tion of  wind-borne  material,  the  prevalence  of  mild  wave  action 


CORRELATION  OF  THE  MARINE  AND  FLUVIAL  CYCLES     245 


246  DEVELOPMENT  OF  THE   SHORE  PROFILE 

because  of  offshore  winds,  and  the  existence  of  a  landward  in- 
stead of  a  seaward  bottom  current,  would  all  tend  to  retard  cliff 
recession.     The  absence  of  great  storm  waves  in  low  latitudes, 
and  the  presence  of   coral  building  polyps,   constitute  special 
factors  which  would  have  to  be  taken  into  consideration  in  any 
attempt  to  compare  shore  development  about  tropical  deserts 
with  that  about  the  more  humid  land  areas  of  higher  latitudes. 
Independence  of  Marine  and  Fluvial  Cycles.  —  It  is  imports^nt 
to  remember  that  there  is  no  necessary  connection  between  the 
stages  of  development  of  a  shoreline  and  the  stages  of  develop- 
ment of  the  land  mass  which  it  borders.     Each  one  develops 
independently,  the  one   under   the   influence   of  marine  forces, 
the  other  under  the  influence  of  subaerial  forces.     If  both  begin 
their  evolution  at  the  same  time,  the  shoreline  maj^  be  young 
while  the  land   mass  is  in  a  youthful  stage  of  development; 
and   it   may   even   happen   that   both   attain   full   maturity  at 
about  the  same  time.     But   this   is   not   a   necessary,  and   not 
even  a  common  relation.     When  a  young  shoreline  of  submer- 
gence is  produced  by  the  partial  submergence  of  a  mature  land 
mass,  the  land  mass  remains  mature  throughout  the  youth  of 
the  shorehne,  for  a  shght  submergence,  which  is  sufficient  to  ini- 
tiate an  entirely  new  cycle  of  shoreline  development,  produces 
scarcely  any  appreciable  effect  upon  the  main  mass  of  the  land. 
The  sea  invades  the  lower  reaches  of  the  valleys,  and  the  remain- 
ing lower  courses  of  some  rivers  may  have  their  gradients  slightly 
reduced  if  delta  building  takes  place  at  the  bay  heads.     But 
the  land  mass  as  a  whole  still  consists  of  high  hills  and  ridges 
separated  by  deep-cut  branching  streams;   it  is  still  a  maturely 
dissected  region,  and  its  cycle  of  erosion  continues  without  any 
real  interruption  toward  the  ultimate  goal  of  planation. 

The  principle  here  involved  is  an  important  one,  and  since 
there  is  not  complete  agreement  concerning  it,  a  further  word  of 
explanation  is  in  order.  Davis  has  at  different  times  presented 
the  idea  that  any  change  of  level  introduces  a  new  cycle  of  land- 
mass  development.  According  to  his  interpretation  the  land 
mass  which  was  mature  before  depression  had  inaugurated  a  new 
cycle  of  shoreline  development,  would  become  young  in  a  new- 
cycle  of  subaerial  erosion  as  soon  as  the  change  of  level  occurred. 
In  speaking  of  such  changes  of  level  he  writes,  "  The  previous 
cycle  (of  land  dissection)  is  thus  cut  short  and  a  new  cycle  is 


INDEPENDENCE  OF   MARINE  AND   FLUVIAL   CYCLES      247 

entered  upon  "*^;  and  again,  "  a  cycle  is  interrupted  when  the 
land  mass  rises  or  sinks,  or  when  it  is  warped,  twisted,  or  broken. 
Like  accidents,  interruptions  may  happen  at  any  stage  of  de- 
velopment. It  is  then  convenient  to  say  that  the  sequential 
form  attained  in  the  first  incomplete  cycle  shall  be  called  the 
initial  form  of  the  new  cycle,  into  which  the  region  enters,  more 
or  less  tilted  or  deformed  from  its  former  shape"^^. 

There  are  certain  theoretical  considerations  which  favor  such 
an  interpretation  as  is  outlined  in  the  above  quotations;  but  it 
seems  to  the  writer  that  numerous  practical  difficulties  outweigh 
these  considerations.  Under  the  proposed  scheme  a  submature 
plateau  with  large,  flat-topped  inter-stream  areas,  a  mature 
plateau  with  sharp-crested  ridges  separated  by  V-shaped  valleys, 
and  an  old  plateau  characterized  by  low  and  gently  undulating 
topography,  would  all  have  to  be  called  "  young  "  in  case  each 
had  been  slightly  depressed  and  not  much  modified  since.  Forms 
of  totally  different  appearance,  and  typically  characteristic  of 
three  distinct  stages  of  normal  plateau  dissection,  would  be 
grouped  together  as  in  the  same  stage  of  development  in  the  new 
cycle  due  to  submergence.  .  An  observer  in  the  interior  would 
never  be  able  to  tell  the  stage  of  development  of  land  forms  until 
he  had  visited  the  coast  to  make  sure  that  neither  emergence  nor 
submergence  had  introduced  a  new  cycle,  the  recognizable  effects 
of  which  were  limited  to  the  coastal  zone.  Indeed,  he  would 
find  that  practically  all  land  masses  are  young  in  the  current 
cycle,  for  emergence  or  submergence  has  occurred  on  most 
coasts  within  a  period  geologically  so  recent  that  little  modifi- 
cation of  surface  forms  has  occurred  since.  The  terms  young, 
mature,  and  old  would  no  longer  be  aids  to  an  appreciation  of 
significant  differences  in  land  forms,  and  the  strongest  argument 
for  interpreting  the  surface  features  of  the  earth  in  terms  of 
their  stages  of  development  would  disappear.  Davis  has  him- 
self in  a  recent  volume*'^  recognized  the  difficulty  of  applying 
strictly  his  earlier  suggestions  regarding  the  terminology  of  the 
cycle  and  has  proposed  to  avoid  the  diflficulty  in  part  by  the  use 
of  circumlocutions  or  explanatory  paraphrases. 

If  it  appears  that  I  have  pushed  an  unimportant  point  to  an 
absurd  extreme,  it  must  be  remembered  that  a  substantial 
agreement  as  to  the  usage  of  the  terms  cycle,  young,  mature,  and 
old  is  absolutely  essential  to  an  intelligent  understanding  of 


248  DEVELOPMENT   OF  THE   SHORE   PROFILE 

land  form  description,  and  that  the  difficulties  I  have  portrayed 
are  the  necessary  and  logical  consequence  of  considering  every 
change  of  level  as  inaugurating  a  new  cycle  of  land-mass  develop- 
ment. If  the  same  mountain  mass  is  to  be  called  mature  by 
one  observer  because  of  the  advanced  stage  of  its  dissection  by 
stream  erosion,  and  young  by  another  observer  who  finds  that 
its  borders  are  slightly  submerged  in  the  sea,  endless  confusion 
must  result.  It  will  scarcely  meet  the  situation  to  say  that  "  a 
mature  mountainovis  region  was  slightly  submerged  and  is  now 
young  in  the  new  cycle,"  for  such  a  double  description  is  too 
cumbrous  to  supply  the  need  for  a  concise,  clear,  and  consistent 
method  of  land-form  description.  One  may,  however,  properly 
say  that  "  a  mature  mountainous  region  was  slightly  submerged, 
and  its  shoreline  is  now  young."  Every  significant  change  of  level 
does  introduce  a  new  cycle  of  shoreline  development;  and  it  is 
evident  that  failure  to  attach  sufficient  importance  to  the  fact 
that  the  cycle  of  shoreline  development  and  the  cycle  of  land- 
mass  development  are  wholly  independent,  and  progress  at  differ- 
ent rates  under  the  influence  of  different  forces,  is  responsible 
for  the  conception  that  every  change  of  level  introduces  a  new 
cycle  of  subaerial  denudation.  The  absence  of  a  clear  cUscrimi- 
nation  between  the  two  cycles  is  especially  noticeable  in  the  con- 
text from  which  the  second  of  the  above  quotations  is  taken"^. 

All  of  the  difficulties  discussed  above  disappear  if  we  adopt 
the  following  as  fundamental  principles  in  land-form  description: 
(1)  The  cycle  of  shoreline  development  and  the  cycle  of  land- 
form  development  are  measurably  independent  as  regards  the 
evolution  of  their  sequential  stages,  and  must  be  treated  as  two 
distinct  cycles.  Both  may  originate  from  the  same  change  of 
level,  their  corresponding  stages  of  development  may  in  some 
instances  be  closely  correlated  especially  near  the  sea,  their 
relative  rates  of  progress  in  a  given  region  may  be  compared, 
and  the  influence  of  one  upon  the  other  may  be  studied;  but 
the  two  distinct  cycles  must  always  be  carefully  discriminated. 
It  might  be  added  that  the  cycle  of  stream  development, 
and  the  cycle  of  land-mass  development  (dissection)  which  are 
very  generally  confused  with  each  other,  as  well  as  with  the 
marine  cycle,  are  likewise  distinct  and  may  progress  at  different 
rates™.  (2)  Emergence  introduces  a  new  cycle  of  shoreline  devel- 
opment, and  will,  if  of  sufficient  magnitude,  introduce  new  cycles 


INDEPENDENCE  OF  MARINE   AND   FLUVIAL   CYCLES      249 

of  stream  development  and  of  land-mass  dissection.  A  slight 
emergence,  especially  if  very  gradual,  will  merely  accelerate  the 
progressive  development  of  the  cycles  of  stream  development 
and  of  land-mass  dissection  already  current,  or  will,  if  re- 
peated, cause  pulsations  of  reinvigorated  stream  cutting  to 
advance  inland  up  the  rivers.  The  result  may  be  minor  topo- 
graphic changes  of  the  highest  importance  to  the  student  of 
past  fluctuations  of  level,  and  these  topographic  records  must  be 
fully  appreciated  and  emphasized.  But  unless  they  are  of  large 
magnitude,  rising  to  the  dignity  of  a  truly  rejuvenated  topog- 
raphy, the  short  episodes  which  they  represent  should  not  be 
dignified  by  the  name  of  cycles.  (3)  Submergence  introduces 
a  new  cycle  of  shoreline  development,  but  submergence  alone 
never  introduces  a  new  cycle  of  stream  development  or  of  land- 
form  dissection.  This  is  because  the  forces  which  cause  stream 
development  and  land-mass  dissection  continue  their  work  as 
before,  in  essentially  the  same  relative  positions  as  before,  even 
though  absolute  altitude  is  different  and  absolute  efficiency  may 
be  more  or  less  modified  by  a  change  in  rainfall.  Bayhead 
deltas  may  form  in  the  drowned  valleys,  the  gradients  of  some 
streams  may  be  diminished  for  a  limited  distance  inland  from 
their  mouths,  and  the  rate  of  erosion  on  adjacent  slopes  may  be 
somewhat  retarded.  But  these  local  and  temporary  effects  have 
no  appreciable  influence  on  the  dissection  of  the  land  as  a  whole, 
and  to  no  extent  do  they  "  rejuvenate  "  it.  In  other  words, 
submergence  does  not  "  determine  a  more  or  less  complete  break 
in  processes  previously  in  operation,  by  beginning  a  new  series  of 
processes  with  respect  to  the  new  baselevel  "^^  and  therefore  does 
not  inaugurate  new  cycles  of  stream  or  land-mass  development. 
It  is  important  that  one  should  distinguish,  not  only  between 
the  shoreline  cycle  and  other  physiographic  cycles,  but  also 
between  stages  in  the  development  of  the  shore  profile  and  stages 
of  shoreline  development.  On  a  shoreline  of  submergence,  for 
example,  it  very  often  happens  that  the  shore  profile  at  certain 
points  becomes  mature  {i.e.,  the  marine  bench  is  pushed  inland, 
the  cliff  weathers  back  to  a  gently  inclined,  soil-covered  slope, 
and  the  shore  profile  of  equilibrium  is  fully  established)  long 
before  the  shoreline  as  a  whole  is  reduced  to  a  comparatively 
simple  line  back  of  the  bayheads.  The  shoreline  in  this  case  is 
still  young,  and  may  even  retain  the  excessive  irregularities  of 


250  DEVELOPMENT  OF  THE   SHORE   PROFILE 

very  early  youth;  but  the  shore  profile  is  mature  at  some  places, 
although  still  young  at  others.  On  the  other  hand,  where  wave 
erosion  is  unusually  effective  the  irregularities  of  a  shoreline  of 
submergence  may  be  quickly  removed,  and  the  shore  outline 
transformed  to  a  line  of  simple  curvature  back  of  the  original 
positions  of  the  bayheads,  at  a  time  when  the  waves  are  still 
actively  undermining  the  marine  cliffs  and  pushing  them  back- 
ward. In  this  case  the  shoreline  is  mature,  but  the  shore  pro- 
file is  young.  It  would  be  quite  proper  to  speak  of  that  part  of 
the  profile  called  the  marine  cliff  as  a  "  young  cHff,"  but  to 
describe  the  shoreline  as  "  young  "  would  be  erroneous. 

The  coast  of  Normandy  exhibits  a  shoreline  of  fairly  simple 
curvature,  bordered  by  very  steep  or  even  vertical  cliffs  of  bare 
rock,  from  which  landslides  often  descend  into  the  rapidly  ad- 
vancing sea.  Here  the  shoreline  is  mature  or  late  mature  while 
the  profile  is  young.  Davis''^  has  described  the  cliffs  of  this  coast 
as  late  mature  ("  spatreife  Kliffe  ");  but  it  is  evident  from  his 
descriptions  and  from  his  characteristically  expressive  diagram 
representing  this  coast,  that  the  expression  "  late  mature " 
reall}^  applies  to  the  shoreline  alone,  while  the  cliffs  along  the 
shoreline  are  marked  by  the  steep  slopes  and  frequent  landslides 
found  only  in  young  cliffs.  Davis  has  himself  given  such  ex- 
cellent accounts  of  the  diverse  features  of  young  and  late  mature 
marine  cliffs  in  other  connections  that  there  can  be  no  doubt  his 
application  of  the  term  "  late  mature  "  to  the  Normandy  cliffs 
was  merely  an  oversight  such  as  is  common  in  physiographic 
literature  where  stages  of  shoreline  development  and  stages  of 
shore  profile  development  are  not  sharply  discriminated. 

Comparative  Rapidity  of  Marine  and  Fluvial  Planation.  — • 
Among  those  who  admit  the  ability  of  unlimited  wave  action 
to  reduce  a  land  mass  to  an  abrasion  platform  below  sealevel, 
there  are  a  number  who  believe  that  fluvial  denudation  takes 
place  so  much  more  rapidly,  that  any  large  land  mass  must 
be  reduced  to  a  peneplane  before  the  waves  could  cut  any 
great  distance  into  the  lands.  This  view  is  well  expressed  by 
Geikie^^  in  these  words:  "  Before  the  sea,  advancing  at  the 
rate  of  ten  feet  in  a  century,  could  pare  off  more  than  a  mere 
marginal  strip  of  land,  between  70  and  80  miles  in  breadth,  the 
whole  land  might  be  washed  into  the  ocean  by  atmospheric 
(meaning  fluvial)  denudation." 


RAPIDITY  OF   MARINE  AND   FLUVIAL   PLANATION      251 

It  is  admittedly  a  difficult  matter  to  find  any  basis  for  an 
adequate  comparison  of  the  relative  rates  of  marine  and  fluvial 
denudation;  but  there  should  be  no  difficulty  in  seeing  that 
Geikie's  comparison  is  based  on  figures  which  enormously  over- 
estimate the  average  rate  of  stream  erosion.  As  a  starting  point 
in  his  calculations  he  takes  the  amount  of  sediment  annually  dis- 
charged by  the  Mississippi  River,  and  computes  that  this  river 
will  lower  the  land  throughout  its  whole  drainage  basin  an  average 
of  1  foot  in  6000  years.  He  cites  other  rivers  which  reduce  their 
drainage  basins  much  more  rapidly,  but  the  rate  just  given  is 
assumed  as  a  conservative  figure  in  calculating  rates  of  fluvial 
denudation.  The  error  consists  in  reckoning  denudation  at  the 
same  rate  throughout  the  entire  fluvial  cycle.  It  is  true  Geikie 
recognizes  that  "  the  last  stages  in  the  demolition  of  a  continent 
must  be  enormously  slower  than  during  earlier  periods  "'''',  but 
he  makes  no  allowance  for  this  fact  in  his  calculations,  except 
to  intimate  that  the  resulting  error  may  be  compensated  for  by 
the  material  removed  in  solution  and  not  figured  in  the  above 
estimate. 

The  Mississippi  River  drains  vast  areas  of  high  mountains  and 
plateaus  whose  steep  slopes  contribute  large  quantities  of  waste 
to  its  upper  branches;  and  extensive  stretches  of  semi-arid 
plains  where  fine-grained  unconsolidated  sediment  is  shed  into 
the  streams  with  enormous  rapidity.  Much  of  the  river's 
drainage  area  has  reached  maturity,  and  its  larger  branches  are 
transporting  heavy  loads  of  debris  on  fairly  steep  profiles  of 
equilibrium.  There  can  be  no  comparison  between  the  amount 
of  material  carried  to  the  sea  by  the  Mississippi  at  the  present 
time,  and  the  amount  which  will  be  carried  when  the  mountains 
and  plateaus  are  worn  lower,  the  stream  gradients  reduced,  the 
rainfall  diminished  because  of  decreasing  relief,  and  the  stream 
volumes  greatly  lessened  because  of  decreased  rainfall  and  in- 
creased evaporation.  The  annual  denudation  under  those  con- 
ditions will  be  but  a  very  small  fraction  of  what  it  is  to-day, 
unless  the  efficiency  of  seolian  denudation  is  enormously  in- 
creased as  the  land  wears  lower.  Instead  of  allowing  4,500,000 
years  for  the  removal  of  the  entire  continent  of  North  America, 
it  is  conceivable  that  it  might  be  nearer  the  truth  to  allow  that 
much  time  for  the  reduction  of  the  surface  by  1  meter  during  the 
latest  stages  of  subaerial  denudation.     Portions  of  tertiary  pcne- 


252 


DEVELOPMENT  OF  THE  SHORE  PROFILE 


X 
X 
X 


O 


CI, 

a 
O 


o 


PROBABILITY   OF  MARINE   PLANATION  253 

planes  which  have  been  exposed  to  erosion  for  a  period  which 
may  be  estimated  as  one  or  more  millions  of  years^^,  not  only 
have  not  been  reduced  nearly  to  sealevel,  but  seem  to  stand 
somewhere  near  the  original  positions  of  the  upland  surfaces. 
Very  many  more  millions  of  years  would  be  required  to  reduce 
these  areas  of  hard  rock  to  a  low-lying  surface  of  fluvial  denu- 
dation. How  much  this  time  might  be  shortened  by  seolian 
erosion  is  prol^lematical;  but  the  combined  action  of  the  sub- 
aerial  forces  could  scarcely  accomplish  the  work  in  so  short  a 
time  as  a  few  million  years. 

It  appears,  therefore,  that  while  it  is  not  possible  to  more  than 
guess  at  the  time  required  for  the  subaerial  denudation  of  a 
continent,  the  advantages  are  not  so  overwhelmingly  in  favor 
of  subaerial  denudation,  and  against  marine  denudation,  as  has 
been  supposed  to  be  the  case.  There  are  indeed,  as  we  have 
already  seen,  certain  marked  advantages  in  favor  of  marine 
planation,  not  the  least  of  which  is  the  slight  rise  of  sealevel,  due 
to  the  infilling  of  sediment  in  the  ocean  basins  and  therefore  nor- 
mal to  the  marine  cycle,  which  brings  the  waves  against  the 
non-resistant  fluvial  deposits  and  residual  hills  of  the  old  land 
mass.  So  far  as  a  priori  reasoning  is  concerned,  we  should 
recognize  the  possibility  that  wave  erosion  may  completely 
plane  away  a  large  land  area  before  the  subaerial  forces  have  had 
time  to  reduce  it  to  sealevel.  Which  forces  have  been  the  more 
effective  in  producing  known  peneplanes  must  be  decided,  if  at 
all,  on  the  characteristics  of  the  peneplanes  themselves,  and  not 
on  the  basis  of  a  priori  arguments. 

Probability  of  Marine  Planation.  —  Several  authors  have  ex- 
pressed the  opinion  that  movements  of  a  land  mass  must  prevent 
extensiv^e  marine  planation  by  repeatedly  forcing  the  waves  to 
begin  anew  the  cycle  of  denudation  at  a  new  level.  This  view 
is  stated  by  Davis  in  the  words:  "  The  sensitiveness  of  a  local 
shoreline  to  changes  in  the  ocean  basin  or  border  all  around  the 
world  makes  extensive  plains  of  marine  abrasion  of  improbal)le 
occurrence  "^^  Emphasis  is  properly  laid  upon  the  fact  that 
marine  erosion  is  restricted  to  a  narrow  vertical  zone  about  the 
margin  of  the  lands,  the  position  of  this  zone  altering  with  every 
change  in  the  relative  level  of  land  or  sea;  whereas  subaerial 
denudation  goes  on  simultaneously  over  the  entire  land  area, 
regardless   of    sealevel    oscillations.      "  A    slight    movement    of 


254  DEVELOPMENT  OF  THE  SHORE   PROFILE 

elevation  usually  sets  the  sea  back  to  begin  its  work  anew  on  the 
seaward  side  of  its  previous  shoreline,  but  such  an  elevation  only 
accelerates  the  work  of  subaerial  denudation  all  over  the  elevated 
region.  The  waves  on  the  seashore  shift  their  line  of  attack 
with  every  slight  vertical  movement  of  the  coastal  region;  but 
the  subaerial  forces  over  large  continental  areas  gain  no  notice 
of  slight  movements  until  a  considerable  time  after  they  have 
been  accomplished,  and  hence  they  perform  their  task  only  with 
reference  to  the  average  attitude  of  the  land  "". 

When  the  theory  of  fluvial  peneplanation  was  first  proposed, 
it  was  objected  that  the  land  could  not  stand  still  long  enough 
to  permit  streams  to  wear  large  areas  nearly  down  to  sealevel. 
The  best  answer  to  this  objection  was  the  finding  of  broad  erosion 
surfaces,  the  characteristics  of  which  indicated  a  fluvial  origin. 
In  like  manner,  we  must  depend  upon  field  evidence  to  settle 
the  question  whether  extensive  planes  or  peneplanes  of  marine 
denudation  have  been  produced  in  the  past.  We  may  fully 
recognize  the  sensitiveness  of  marine  erosion  to  changes  of  level, 
without  denying  the  possibility  of  marine  planation.  If  we 
find  erosion  planes  having  the  characteristics  of  planes  of  marine 
abrasion  rather  than  those  of  subaerial  denudation,  we  may 
reasonably  conclude  that  the  land  can  stand  still  long  enough  for 
waves  to  reduce  a  land  mass  to  a  plane  surface.  The  distinguish- 
ing features  of  planes  and  peneplanes  of  different  origins  have 
been  receiving  more  attention  in  recent  years  than  formerly, 
and  we  may  anticipate  that  discrimination  between  these  sur- 
faces, at  least  where  they  are  fairly  well  preserved,  will  become 
practicable. 

Attention  may  here  be  called  to  a  tendency  to  regard  erosion 
surfaces  which  show  characteristics  of  marine  planation,  as  fluvial 
peneplanes  which  have  been  planed  down  further  by  the  sea. 
It  would  perhaps  be  more  pertinent  to  speak  of  them  as  marine 
planes  or  peneplanes  whose  development  was  favored  by  exten- 
sive subaerial  erosion  of  the  land.  For  when  we  remember  that 
relatively  flat  fluvial  peneplanes  may  have  a  relief  of  several 
hundred  feet  and  that  marine  abrasion  reduces  a  land  mass  many 
feet  below  sealevel,  it  is  evident  that  the  waves  must  perform 
much  work  in  planing  away  a  fluvial  peneplane.  Furthermore, 
marine  abrasion  destroys  the  essential  characteristics  of  fluvial 
denudation,  including  the  extensive  adjustment  of  stream  val- 


PROBABILITY  OF   MARINE  PLANATION 


255 


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256 


DEVELOPMENT  OF  THE  SHORE  PROFILE 


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lej's  to  weak  rock  belts,  which  is  one  of 
the  best  evidences  of  long-continued  fluvial 
action.  Since  both  marine  and  fluvial 
planation  are  possible,  it  is  perhaps  safer, 
in  the  absence  of  evidence  to  the  contrary, 
to  regard  an  erosion  surface  covered  with 
remnants  of  a  marine  veneer  as  a  marine 
plane  or  peneplane,  rather  than  to  make 
the  gratuitous  assumption  that  there  must 
have  been  a  fluvial  peneplane  which  was 
later  planed  off  by  the  waves.  Rapid  de- 
pression of  a  fluvial  peneplane  would  econ- 
omize the  amount  of  wave  work  necessary 
to  produce  the  observed  result^^;  but  one 
is  not  justified  in  assuming  both  fluvial 
peneplanation  and  rapid  submergence  in 
the  absence  of  supporting  evidence. 

It  is  sometimes  assumed  that  a  cover 
of  marine  sediments  is  an  essential  feature 
of  a  marine  plane  or  peneplane''^.  While 
a  thin  marine  veneer  may  be  expected  in 
many  or  even  in  most  cases,  its  presence 
in  any  appreciable  quantity  does  not  seem 
necessary  in  the  later  stages  of  the  marine 
cycle.  A  land  mass  reduced  to  an  abrasion 
platform  surrounded  by  a  continental 
terrace,  as  a  result  of  wave  attack  from 
all  sides,  Avould,  in  the  penultimate  stage, 
have  the  form  of  a  very  flat  cone  with 
the  apex  (a)  where  the  last  land  surface 
was  reduced  (Fig.  40).  Unimpeded  by 
any  further  contributions  of  debris  from 
a  land  area,  wave  erosion  would  proceed 
to  remove  the  veneer  which  might  have 
accumulated  on  the  faintly  conical  plat- 
form, and  to  reduce  the  rock  surface  to 
wave  base  with  no  cover  except  an  in- 
significant amount  of  recently  eroded 
debris  in  transit  to  deeper  water.  As  ex- 
plained  on   a   previous   page,  the  conti- 


ACCIDENTS  DURING  THE   MARINE   CYCLE  257 

nental  shelf  would  then  consist  of  deposited  material  at  the  outer 
borders,  and  a  bare  rock  erosion  surface  within.  Essentially 
the  same  conditions  might  prevail  at  an  earlier  stage,  where  a 
broad  continental  shelf  bordered  a  still  remaining  land  area,  if 
the  supply  of  land  waste  were  very  slow.  When  uplifted  the 
land  area,  abrasion  platform  and  continental  terrace  would 
occupy  the  same  relative  positions  as  the  Older  Appalachian 
Mountains,  the  Piedmont  Belt,  and  the  Atlantic  Coastal  Plain. 
A  very  thin  marine  veneer,  quickly  removed,  might  be  insuffi- 
cient to  superimpose  rivers  upon  transverse  hard  rock  ridges, 
even  when  these  were  worn  down  nearly  to  the  level  of  an  almost 
plane  abrasion  surface;  and  this  partial  initial  adjustment  of 
streams  to  rock  structure  would  be  greatly  increased  during 
further  dissection.  It  is  essential,  therefore,  to  keep  an  open 
mind  as  to  the  possible  origin  of  uplifted  and  dissected  peneplanes 
which  show  no  traces  of  a  former  marine  cover,  and  which  may 
even  show  a  considerable  adjustment  of  stream  courses  to  rock 
structure. 

Interruptions  and  Accidents  During  the  Marine  Cycle.  — 
Davis^"  has  repeatedly  emphasized  the  importance  of  the  "  in- 
terruptions "  and  "  accidents  "  which  frequently  occur  in  the 
fluvial  cycle.  Similar  events  diversify  the  history  of  the  marine 
cycle.  We  have  already  seen  that  elevation  may  end  the  prog- 
ress of  the  marine  cycle  at  a  given  level  by  raising  the  abrasion 
platform  and  continental  terrace,  or  parts  of  them,  above  the 
reach  of  the  waves.  Subsidence,  if  rapid,  may  produce  the  same 
effect  by  lowering  the  platform  and  terrace  far  beneath  the 
lowest  limits  of  wave  activity.  Slow,  progressive  subsidence 
may  simply  hasten  the  development  of  the  marine  cycle  by 
constantly  deepening  the  water  offshore  and  thus  facilitating 
wave  erosion.  It  would  seem,  however,  that  any  considerable 
help  from  subsidence  would  demand  rather  rapid  sinking,  in 
order  to  keep  the  water  offshore  continually  and  appreciably 
deeper.  As  subsidence  progresses  the  inner  margin  of  the  con- 
tinental terrace  advances  landward,  so  that  the  outer  margin  of 
the  abrasion  platform  is  progressively  overlapped  by  a  wedge 
of  marine  deposits  which  thicken  seaward  (Fig.  41). 

Accidents  may  occur  during  any  part  of  the  marine  cycle,  and 
locally  interfere  for  a  time  with  the  normal  development  of  the 
shore  profile.      Glaciers  may  excavate  deep  troughs  far  below 


258 


DEVELOPMENT  OF  THE   SHORE   PROFILE 


wave  base.  Volcanic  eruptions  may  build  cones  upon  the  con- 
tinental shelf,  the  summits  of  the  cones  possibly  rising  above 
sealevel.  But  in  course  of  time  the  submarine  troughs  will  be 
filled  with  sediment,  the  volcanoes  will  be  removed  by  wave 
erosion,  and  the  development  of  the  shore  profile  will  proceed 


Fig.  41.  —  Overlapping  of  marine  deposits  upon  the  abrasion  platform  of  a 
slowly  subsiding  land  mass. 

as  before.  A  longer-enduring  departure  from  the  ideal  scheme 
will  occur  if  a  strong  and  deep  ocean  current  abrades  the  bottom 
long  enough  to  reduce  it  below  wave  base.  But  even  this  ac- 
cident must  be  corrected  as  the  removal  of  land  masses  and  the 
reduction  of  shallows  to  wave  base  make  concentrated  current 
action  impossible. 

SHORELINES  OF  EMERGENCE 

Initial  Stage.  —  In  the  typical  shoreline  of  emergence  the 
water  margin  comes  to  rest  against  the  exposed  sea  floor.  Under 
normal  conditions  this  floor .  consists  of  an  abrasion  platform 
and  continental  terrace,  the  smooth  surface  of  which  is  inter- 
sected by  the  plane  of  the  sea  surface  to  form  a  very  simple 
shoreline.     Inland  the  land  rises  very  gently  in  the  form  of  a 


Fig.  42.  —  Elements  of  the  profile  of  a  shoreline  of  emergence. 

smooth  marine  plane  or  coastal  plain,  as  the  case  may  be;  sea- 
ward the  bottom  slopes  downward  with  the  same  gentle  incli- 
nation, giving  shallow  water  for  a  long  distance  offshore. 

Davis  has  briefly  outlined  the  broader  features  in  the  develop- 
ment of  shorelines  of  emergence  on  the  assumption  that  emer- 
gence is  relatively  rapid  and  is  then  followed  by  a  still-stand  of 
the  land.  Waves  attack  the  initial  shoreline  with  results  to  the 
profile  which  are  at  first  similar  to  those  produced  at  the  shore- 
line of  submergence.    A  marine  bench  (Fig.  42,  h)  is  cut,  a  marine 


YOUNG   STAGE  259 

cliff  (c)  produced,  and  a  shoreface  terrace  (a)  built.  The  bench 
may  be  covered  by  a  thin  beach  deposit.  But  in  two  respects  the 
development  of  the  profile  of  a  shoreline  of  emergence  is  signifi- 
cantly different  from  that  previously  described.  In  the  first 
place,  only  small  waves  can  reach  the  shore,  because,  according 
to  the  law  of  wave  breaking  set  forth  in  Chapter  I,  large  waves 
break  when  they  enter  water  whose  depth  is  approximately  equiv- 
alent to  the  wave  height;  and  this  must  occur  well  out  from 
land  in  the  case  of  shorelines  of  emergence.  Accordingly,  the 
marine  bench  is  shallow,  and  the  marine  cliff  is  low,  both  being 
pushed  slowly  into  a  low  h'ing  plain  by  weak  waves.  Because 
of  its  insignificant  size,  the  cut  made  by  the  waves  during  this 
earliest  stage  of  development  is  often  spoken  of  as  a  nip  in  tlie 
edge  of  the  land.  The  nip  is  frequently  preserved  from  further 
change  for  a  long  period  of  time  by  the  development  of  an  aff- 
shore  har^(B),  which  is  a  second  feature,  characteristic  of  the 
shoreline  of  emergence,  not  found  along  typical  shorelines  of  sub- 
mergence. 

As  is  elsewhere  pointed  out,  it  may  well  happen  that  pro- 
gressive emergence  prevents  the  formation  of  a  distinct  nip  on 
the  "mainland  shore  until  after  the  offshore  bar  has  been  formed. 
If  the  levels  of  land  and  water  then  become  stationarj%  and  the 
lagoon  is  sufficiently  broad  and  deep,  lagoon  waves  may  produce 
a  nip  of  later  date  than  the  bar.  On  the  other  hand,  if  emergence 
continues,  or  if  submergence  intervenes,  or  if  the  lagoon  waves 
are  too  feeble,  the  nip  may  be  entirely  lacking.  Whether  or  not 
a- nip  is  formed,  the  shoreline  is  past  its  initial  stage  and  entered 
upon  the  stage  of  youth  as  soon  as  the  offshore  bar  is  built  up 
above  the  water  surface. 

Young  Stage,  —  Under  the  various  names  of  barrier  beach, 
sand  reef,  and  offshore  barrier,  the  offshore  bar  has  been  described 
as  a  continuous  narrow  ridge  of  sand,  lying  some  distance  out 
from  shore.  Its  seaward  side  has  the  normal  beach  profile  of 
equilibrium,  and  its  crest  rises  a  few  feet  above  high  tide  level. 

The  precise  manner  in  which  the  offshore  bar  originates  is 
not  definitely  known.  Various  theories  advanced  to  account 
for  its  development  are  considered  at  length  in  a  later  chapter 
but  onl}'  two  deserve  special  mention  here.  The  first  is  that 
of  Gilbert,  which  is  based  on  the  belief  that  the  material  of  the  bar 
consists  of  "  shore  drift,"  which  is  being  moved  parallel  to  the 


260  DEVELOPMENT  OF  THE  SHORE  PROFILE 

coast  by  longshore  currents.  ''  The  most  violent  agitation  of  the 
water  is  along  the  line  of  breakers;  and  the  shore  drift,  depending 
upon  agitation  for  its  transportation,  follows  the  line  of  the 
breakers  instead  of  the  water  margin.  It  is  thus  built  into  a  con- 
tinuous outlying  ridge  at  some  distance  from  the  water's  edge."^^ 
De  Beaumont^-  would  derive  the  material  of  the  bar  from  the 
offshore  deposits,  by  direct  wave  action.  Davis,  who  follows  de 
Beaumont,  states  the  theory  thus:  ''  When  waves  roll  in  upon  a 
shelving  shore,  much  of  their  energy  is  expended  on  the  bottom. 
Between  the  line  of  their  first  action  far  offshore  and  their  final 
exhaustion  on  the  coast,  there  must  be  somewhere  a  zone  of  maxi- 
mum action.  This  zone  must  lie  farther  seaward  when  large 
storm  waves  roll  in  than  when  the  sea  is  slightly  ruffled  in  fair 
weather.  .  .  .  Here  the  bottom  is  deepened;  the  coarser  particles 
are  moved  landward,  forming  a  shoal  and  in  time  a  bar  inclosing 
a  lagoon;  while  the  finer  particles  are  moved  seaward,  where  they 
are  distributed  in  moderate  thickness  over  a  considerable  area."^ 
Conformable  to  these  two  theories,  Gilbert  illustrates  his  idea  of 
the  offshore  bar  by  a  section  which  shows  the  bar  deposit  resting 
on  the  unbroken  surface  of  an  inclined  sea-bottom;  whereas  in 
Davis's  illustrations  the  sea-bottom  is  represented  as  deeply 
eroded  by  the  waves  which  used  the  eroded  materials  to  build 
the  bar.  In  Gilbert's  opinion  the  offshore  bar  is  "  absolutely  de- 
pendent on  shore  drift  for  (its)  existence.  If  the  essential  con- 
tinuous supply  of  moving  detritus  is  cut  off,  .  .  .  the  structure 
(is)  demolished  by  the  waves  which  formed  it  "**.  According  to 
Davis,  offshore  bars  "  might  be  developed  essentially  under  the 
control  of  on-  and  offshore  action  alone  "^^ 

Without  pausing  to  discuss  the  relative  merits  of  these  two 
theories  at  this  time,  we  may  note  that  the  further  development 
of  the  shore  profile  would  be  essentially  the  same  in  either  case. 
The  profile  of  the  seaward  side  of  the  bar  is  a  profile  of  equilib- 
rium which  varies  with  variations  in  the  waves  and  other  forces 
which  affect  the  shore,  in  the  manner  already  fully  described. 
Beach  materials  are  heaped  upon  the  backshore  (d)  one  day,  and 
dragged  out  to  form  a  shoreface  terrace  (a')  the  next.  Vigorous 
wave  action  cuts  into  the  sea-bottom  to  form  a  marine  bench  (6')> 
while  the  top  of  the  bar  or  the  sand  dunes  upon  its  crest  may  have 
a  low  but  distinct  marine  cliff  (c')  marking  the  upper  limit  of 
the  shore. 


YOUNG  STAGE  261 

Normal  development  involves  slow  retrogression  of  the  shore- 
line, as  the  grinding  of  the  beach  materials  to  fine  silt  permits 
their  removal  in  suspension  to  deep  water,  or  as  seaward  bottom 
currents  drag  coarser  debris  from  the  face  of  the  bar  down  the 
inclined  slope  of  the  bottom  toward  the  edge  of  the  continental 
terrace.  But  the  retrograding  process  does  not  necessarily  in- 
volve the  rapid  removal  of  the  bar.  The  material  lost  from  the 
bar  in  the  ways  described  above  may  be  compensated  for  by 
material  freshly  cut  from  the  sea-bottom  during  the  landward 
cutting  of  the  marine  bench.  Storm  waves  hurl  debris  over  the 
crest  of  the  bar  to  its  back  side,  and  the  overwash  of  waves 
carries  much  additional  material  down  its  landward  slope. 
Wind-blown  sands  still  further  assist  this  landward  building. 
All  these  factors  combined  may  be  sufficient  to  build  up  the 
inner  side  of  the  bar  as  fast  as  the  outer  side  is  cut  away,  in 
which  case  the  bar  will  retreat  bodily  toward  the  coast  without 
any  marked  change  in  its  average  width. 

Between  the  offshore  bar  and  the  mainland  lies  a  narrow  strip 
of  shallow  water,  called  the  lagoon  (L),  whose  weak  waves  faintly 
cliff  the  lagoon  shores,  often  at  a  lower  level  than  the  initial  nip. 
Tidal  currents  bring  fine  sediments  from  the  surf-beaten  outer 
side  of  the  bar,  to  deposit  them  in  the  quiet  water  of  the  lagoon, 
which  also  receives  some  stream-brought  sediment  from  the  lands, 
wind-blown  sands  from  the  beaches  and  dunes  of  the  bar,  and 
debris  eroded  from  the  lagoon  shores  by  the  waves.  In  course 
of  time  these  sediments  may  build  the  floor  of  the  lagoon  up  to 
such  a  level  that  salt  marsh  vegetation  can  take  possession  in  the 
manner  described  by  Shaler^*'  in  his  oft-quoted  paper  on  the  "  Sea 
Coast  Swamps  of  the  Eastern  United  States,"  and  so  transform 
the  lagoon  into  a  salt  marsh..  It  must  not  be  supposed,  however, 
that  all  salt  marshes  back  of  offshore  bars  have  had  the  history 
outlined  by  Shaler;  for,  as  will  be  shown  in  a  later  chapter,  the 
typical  salt  marshes  of  the  Atlantic  Coast  have  been  formed  in 
an  entirely  different  manner. 

As  the  retrograding  of  the  offshore  bar  continues,  its  sands  and 
gravels  are  driven  in  over  the  marsh  surface.  The  enormous 
weight  of  the  bar  compresses  the  peat  and  other  marsh  deposits, 
which  later  outcrop  on  the  seaward  side  of  the  bar  near  or  below 
low-tide  level,  and  thus  bear  witness  to  the  retrograde  move- 
ment of  the  outer  shoreline.     During  all  this  movement  the 


262      DEVELOPMENT  OF  THE  SHORE  PROFILE 

profile  of  equilibrium  is  maintained  as  perfectly  as  the  varying 
conditions  will  permit.  The  bench  is  deepened  as  well  as  cut 
landward,  and  its  seaward  edge  grades  imperceptibly  into  a 
constantly  broadening  abrasion  platform.  Erosion  products  ac- 
cumulate in  a  continental  terrace  farther  seaward.  At  length 
the  bar  is  driven  upon  the  mainland,  the  marsh  or  lagoon  is  ex- 
tinguished, and  larger  waves  working  on  a  steeper  profile  attack 
the  coast  where  long  before  small  waves  on  the  gently  sloping 
initial  profile  cut  the  less  prominent  nip.  The  shore  profile  is 
now  thoroughly  mature. 

-  It  is  not  necessary  that  the  offshore  bar  should  begin  to  retreat 
as  soon  as  formed.  Larger  storm  waves  may  build  successive 
additional  bars  in  deeper  water  on  the  seaward  side  of  those 
formed  earlier;  but  prograding  of  the  shoreline  from  this  cause 
can  proceed  to  a  very  limited  extent  only,  and  the  extensive 
series  of  "  beach  ridges  "  often  attributed  to  this  action  must  be 
explained  in  some  other  manner.  One  other  explanation  in- 
volves the  supply  of  large  volumes  of  debris  by  longshore  cur- 
rents, which  will  cause  long-continued  prograding  in  the  manner 
already  explained  for  shorelines  of  submergence.  If  the  long- 
shore currents  supply  just  enough  debris  to  make  good  the  loss 
from  wave  erosion,  attrition,  and  removal,  the  shoreline  will 
remain  stationary. 

Mature  and  Old  Stages.  — ■  Whether  or  not  the  offshore  bar 
is  prograded  for  a  period,  retrograding  must  inevitably  replace 
the  temporary  forward  movement  in  the  course  of  time,  and 
the  shoreline  be  driven  back  upon  the  mainland.  Maturity 
begins  when  the  lagoon  or  marsh  is  extinguished,  and  the  waves 
have  begun  their  real  attack  upon  the  coast.  From  this  time 
on  there  are  no  features  of  shore  profile  development  which 
differ  in  any  essential  respect  from  the  mature  and  old  profiles 
on  shores  of  submergence.  As  both  these  stages  of  profile  devel- 
opment have  been  fully  discussed  in  connection  with  shorelines 
of  submergence,  we  may  dismiss  them  without  further  consid- 
eration. 

NEUTRAL   SHORELINES 

The  successive  stages  of  development  in  the  profiles  of  neutral 
shorelines  involve  little  that  is  novel  save  in  matters  of  detail. 
Marine  erosion  of  delta  shorelines,  alluvial  fan  shorelines,  and 


NEUTRAL  SHORELINES  263 

outwash  plain  shorelines  would  give  stages  resembling  those  in 
the  profile  of  shorelines  of  emergence,  except  that  the  offshore 
bar  stage  need  not  necessarily  be  represented  in  case  the  sea- 
ward portion  of  the  profile  descends  too  abruptly  into  deep 
water. 

The  typical  delta  consists  of  two  main  portions,  a  subaerial 
plain  and  a  subaqueous  plain,  separated  by  a  steeper  wave-cut 
slope  to  which  Barrell'*^  originally  gave  the  name  "shore  face." 
The  comparatively  steep  frontal  slope  of  the  delta  may  thus  be 
far  from  the  shoreline,  as  in  the  case  of  the  Nile  delta,  and  is 
unrelated  to  the  true  delta  shore  profile.  The  shoreface,  on 
the  other  hand,  is  the  steeper,  landward  portion  of  the  shore 
profile  of  equilibrium,  of  which  the  profile  of  the  gently  sloping 
subaqueous  plain  is  the  seaward  continuation.  It  should  be 
noted  that  the  outer  margin  of  the  subaqueous  plain,  where  it 
joins  the  steeper  frontal  slope  of  the  delta,  does  not  mark  the 
position  of  wave  base,  as  most  writers  erroneously  assume.  It 
may  mark  the  seaward  end  of  the  profile  of  equilibrium  in  any 
given  section,  the  equilibrium  referred  to  being  the  balance 
between  the  power  of  the  waves  on  the  one  hand,  and  the  work 
they  must  accomplish  in  transporting  debris  on  the  other.  Stop 
the  addition  of  sediment  to  the  delta  for  a  time,  and  the  waves 
will  slowly  reduce  the  submarine  plain,  including  its  outer  mar- 
gin, to  a  still  lower  level.  Where  the  surface  of  the  water 
body  in  which  a  delta  is  built  has  recently  been  raised  or  lowered, 
the  outer  margin  of  the  subaqueous  delta  plain  is  not  only 
unrelated  to  wave  base,  but  is  also  unrelated  as  yet  to  the  nor- 
mal profile  of  equilibrium  for  the  new  conditions.  Wave  base 
is  an  imaginary  horizontal  plane  marking  the  lowest  limit  of 
effective  wave  erosion  in  a  given  water  body.  It  is  highly 
improbable  that  the  seaward  margin  of  any  present  day  delta 
or  shore  terrace  coincides  with  that  imaginary  plane,  just  as  it 
is  highly  improbable  that  any  present  land  surface  coincides  with 
the  imaginary  subaerial  baselevel  plane. 

Neutral  volcano  shorelines  would  have  the  same  profile  de- 
velopment as  slopes  of  corresponding  steepness  on  shorelines  of 
submergence.  Coral  reef  shorelines  have  one  striking  peculi- 
arity, in  that  they  depend  on  organic  as  well  as  on  inorganic 
forces  for  their  history.  Vigorous  coral  growth  may  indefi- 
nitely  postpone   the   developmental   stages   of   the  reef  under 


264 


DEVELOPMENT  OF  THE  SHORE  PROFILE 


marine  erosion,  and  may  even  for  a  long  period  build  the  reef 
forward  into  the  sea  despite  the  most  vigorous  wave  attack. 

Fault  shorelines  deserve  more  than  passing  notice  because  of 
certain  novel  features  which  they  present  both  in  the  initial  and 
in  later  stages.  If  the  hade  of  the  fault  plane  is  steep  and  the 
seaward  block  drops  well  below  sealevel,  in  the  initial  stage 
the  sea  will  come  to  rest  against  a  steep  cliff,  the  fault  scarp 
(a^a-,  Fig.  43)  which  descends  abruptly  into  deep  water.  This 
initial  stage  may  persist  for  an  abnormally  long  period  of  time, 


^ao/ 


^"^^^^x 


Fig.  43.  —  Stages  in  the  development  of  the  shore  profile  of  a  fault  coast. 

due  to  two  important  facts.  In  the  first  place,  as  we  have 
already  seen  in  an  earlier  chapter,  waves  approaching  a  vertical 
or  nearly  vertical  wall  rising  out  of  deep  water  are  reflected 
back  without  developing  any  great  erosive  power;  and  in  the 
second  place,  where  the  water  is  deep  close  to  shore  the  waves 
cannot  arm  themselves  with  any  tools  with  which  to  facilitate 
their  attack  upon  the  land.  Rock  fragments  weathering  from 
the  face  of  the  cliff  descend  at  once  to  deep  water,  beyond  the 
reach  of  effective  wave  action.  If  the  cliff  is  composed  of  very 
resistant  rock  which  yields  but  slowly  to  the  forces  of  weather- 
ing, the  initial  profile  may  long  remain  practically  unaltered. 

In  the  course  of  time,  weathering  of  the  cliff  face  causes  it  to 
retreat  and  leads  to  the  accumulation,  at  its  base,  of  a  submarine 
talus  (60-  Two  important  consequences  follow.  Wave  reflec- 
tion is  less  perfect  and  hence  the  waves  develop  greater  erosive 


COMPOUND  SHORELINES  265 

power  on  the  more  sloping  surface  of  the  talus;  at  the  same  time 

the  waves  become  armed  with  the  talus  debris,  which  is  hurled 

against  the  cliff  face  with  ever-increasing  force.     Under  these 

favorable  conditions  the  retrograding  of  the  cliff  face  may  be 

so  accelerated  as  to  give  it  a  steeper  slope  (c-)  than  it  possessed 

a  short  time  before  {b~),  while  the  prograding  of  a  true  shoreface 

terrace  (c^)  replaces  the  former  talus  growth.     From  this  time 

forth  the  shore  profile  develops  as  in  the  case  of  shorelines  of 

submergence. 

I    COMPOUND    SHORELINES 

The  name  "  compound  shoreline  "  has  been  applied  to  a  shore- 
line which  shows  with  more  or  less  equal  prominence  features 
characteristic  of  at  least  two  of  the  three  simple  classes  of  shore- 
lines. The  best  examples  of  compound  shorelines  exhibit  the 
irregular  pattern  of  drowned  valleys  in  combination  with  a 
smooth  and  gently  sloping  sea-bottom  from  which  an  offshore 
bar  usually  rises  to  the  surface.  There  is  reason  to  believe  that 
in  such  cases  extensive  emergence  takes  place  first,  and  that  later 
moderate  submergence  drowns  the  valleys  carved  in  the  emerged 
coast.  Were  submergence  to  occur  first,  it  is  probable  that  par- 
tial emergence  soon  after  would  find  the  sea-bottom  still  possessed 
of  its  former  irregularities  to  such  a  degree  that  the  new  shoreline 
would  still  be  a  typical  shoreline  of  submergence,  with  its  essen- 
tial characteristics  little  affected  by  the  uplift  which  operated 
merely  to  reduce  the  amount  of  submergence ;  whereas  emergence 
a  long  time  after  would  reveal  a  well-smoothed  sea-bottom  and 
give  a  typical  shoreline  of  emergence.  Compound  shorelines,  of 
which  the  North  Carolina  coast  is  a  typical  example,  may  there- 
fore be  regarded  as  presumptive  evidence  in  favor  of  emergence 
followed  by  partial  submergence. 

The  character  of  the  initial  profile  of  a  compound  shoreline 
of  the  North  Carolina  type  will  depend  partly  upon  the  amount 
of  dissection  which  the  emerged  area  experienced  previous  to 
the  partial  submergence,  and  partly  upon  where  the  profile  is 
tak^n.  If  dissection  was  limited  to  areas  adjacent  to  the  main 
streams,  and  the  profile  is  located  so  as  to  lie  wholly  in  an  undis- 
sected  inter-stream  area,  it  will  not  differ  from  the  initial  profile 
of  the  ordinary  shoreline  of  emergence,  providing  no  offshore  bar 
has  formed.  When,  however,  an  offshore  bar  forms  before  sub- 
mergence changes  the  shoreline  to  the  compound  type,  and  this 


266  DEVELOPMENT  OF  THE   SHORE  PROFILE 

bar  is  built  up  to  the  surface  as  submergence  progresses,  what 
may  be  called  the  initial  profile  of  the  compound  shoreline  will 
resemble  the  young  profile  of  the  shoreline  of  emergence,  in 
which  the  bar  is  a  prominent  feature.  If  dissection  was  so 
extensive  that  submergence  everywhere  brings  the  water  to 
rest  against  relatively  steep  valley  sides,  or  if  the  profile  is  so 
located  as  to  cross  the  shoreline  of  a  slightly  dissected  and 
embayed  plain  within  the  limits  of  one  of  the  drowned  valleys, 
the  initial  profile  of  the  compound  shoreline  will  dift^er  from  the 
young  profile  of  a  normal  shoreline  of  emergence  in  having  a 
steeper  slope  at  the  water  line,  deeper  water  offshore,  and  a 
more  irregular  bottom  for  a  limited  distance  seaward  (Fig.  44). 


Fig.  44.  —  Profile  of  a  shoreline  of  emergence  when  sealevel  is  at  a,  changed 
to  profile  of  a  compomid  shoreline  when  submergence  brmgs  the  sealevel 
to  h  and  faciUtates  the  landward  migration  of  the  offshore  bar. 

In  case  submergence  is  so  rapid  or  so  extensive  as  to  destroy 
the  original  offshore  bar,  no  new  bar  will  form  on  the  submerged 
irregular  surface  of  the  dissected  land  mass  (unless  the  hills  of 
the  land  were  of  such  very  moderate  relief  as  to  constitute  prac- 
tically a  level  plain),  and  we  will  have  a  normal  shoreline  of  sub- 
mergence instead  of  a  compound  shoreline. 

During  submergence  the  offshore  bar  may  be  driven  landward 
by  the  larger  waves  which  are  admitted  by  the  deepening  water 
offshore.  The  small  waves  in  the  lagoon  will  faintly  cliff  the 
lagoon  shores,  and  currents  will  proceed  to  smooth  out  the  in- 
equalities of  the  bottom  by  distributing  the  wave-eroded  debris 
and  the  sediments  brought  in  by  tides  and  rivers.  The  further 
development  of  the  shore  profile  will  be  similar  to  that  of  the 
ordinary  shoreline  of  emergence. 

On  a  compound  shoreline  combining  the  features  of  a  fault 
shoreline  with  those  of  a  shoreline  of  submergence  a  profile 
through  one  of  the  drowned  valley  sections  will  have  in  general 
the  same  developmental  history  as  the  normal  profile  of  a  shore- 
line of  submergence.  A  profile  through  a  typical  portion  of  the 
fault  scarp  will  pass  through  the  sequential  stages  already  de- 
scribed for  normal  fault  shorelines. 


RESUME  207 

RESUME 

We  have  now  traced  the  history  of  the  shore  profile  from  its 
initial  to  its  ultimate  stage.     The  characteristics  of  the  profile 
in  all  the  different  stages  of  development  and  in  the  several 
classes  of  shorelines  have  been  fully  considered,  and  shore  profile  * 
development  has  been  compared  with  the  development  of  stream 
profiles.     This  study  has  led  us  to  certain  important  conclusions, 
which  must  have  an  important  bearing  upon  all  investigations 
of  marine  erosion.     Thus,  it  has  been  shown  that  a  shore  profile 
of  equilibrium  is  early  established,  the  maintenance  of  which  is 
accompanied  by  constant  loss  of  del^ris  and  consequent  recession 
of  the  shoreline.     The  changes  in  this  profile,  which  have  given 
rise  to  so  much  misunderstanding  on  the  part  of  many  observers, 
are  due  to  temporary  changes  in  the  balance  of  the  shore  forces, 
and  are  of  small  importance  as  compared  with  the  general  cycle 
of  -shore  development.     Of  very  great  importance  is  the  fact  that 
long-continued  wave  action  must  reduce  broad  land  areas  to  a 
plane,  or  at  least  to  a  peneplane,  of  marine  denudation.     We 
have  found  that  such  a  plane  or  peneplane  may  be  produced 
without  progressive  subsidence;    that  rapid  wave  cutting  is  no 
proof  of  a  change  in  the  relative  level  of  land  and  sea:   and  that 
while  subaerial  denudation  may  temporarily  embarrass  marine 
abrasion  by  delivering  much  sediment  to  the  sea  margins,  ulti- 
mate marine  planation  cannot  thus  be  prevented.     A  compari- 
son of  the  relative  rapidity  of  marine  and  fluvial  planation  indi- 
cates that  the  widespread  opinion  in  favor  of  the  greater  efficiency 
of  fluvial  erosion  rests  upon  an  inadequate  basis,  and  that  marine 
forces  may  really  be  able  to  reduce  a  large  land  mass  to  a  peneplane 
more  rapidly  than  can  stream  erosion.     Whether  the  land  stands 
still  long  enough  for  such  a  result  to  be  effected  by  the  waves,  is 
a  question  which  cannot  be  answered  on  a  -priori  grounds  and 
which  depends  upon  careful  and  unprejudiced  study  of  actual 
peneplanes  for  its  solution.     In  prosecuting  such  study  it  is  essen- 
tial to  remember  that  neither  the  absence  of  marine  sediments 
nor   the   presence  of  a  certain  degree  of  stream  adjustment  is 
conclusive  evidence  in  favor  of  a  subaerial  as  opposed  to  a  marine 
origin  for  a  given  peneplane.     The  marine  cycle  of  erosion  is 
subject  to  interruptions  and  accidents.,  the  occurrence  of  which 
does  not,  however,  affect  the  general  principles  controlling  the 
cycle  of  shore  development  under  normal  conditions. 


268  DEVELOPMENT  OF  THE   SHORE   PROFILE 


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No.  29,  p.  48,  1900. 

40.  NussBAUM,    F.     Uber    die    Entstehung    der    Norwegischen    Fjeldland- 

schaften,  Fjorde,   und  Scharen.     Mitt,   naturf.   Ges.   Bern,   233-258, 
1909. 

41.  Nansen,  Fridtjof.     The  Bathymetrical  Features  of  the  North  Polar 

Seas.     The  Norwegian  North  Polar  Expedition.     IV,  Art.  XIII,  pp. 
20,  90,  1904. 

42.  Gushing,  S.  W.     The  East  Coast  of  India.     Bull.  Amer.  Geog.  Society. 

XLV,  87-88,  1913. 

43.  Gushing,  S.  W.     Personal  communication. 

44.  Nansen,  Fridtjof.     The  Bathymetrical  Features  of  the  North  Polar 

Seas.     The  Norwegian  North  Polar  Expedition.     IV,  Art.  XIII,  pp. 
151-153,  193,  1904. 

45.  Ibid.,  pp.  105-112. 

46.  Ibid.,  p.  112. 

47.  RiCHTHOFEN,  F.  VON.     China.     II,  768-773,  Berlin,  1882. 

48.  RiCHTHOFEN,  F.  VON.     Fiihrer  fiir  Forschungsreisende,  p.  333,  Hanover, 

1901. 

49.  Martonne,   E.   de.     Traite    de    Geographic   Physique,    p.   676,   Paris, 

1909. 

50.  Lapparent,  a.  DE.      Traite  de  Geologie;   Phenomenes  Actuels,  p.  238, 

Paris,  1900. 

51.  Lapparent,  A.  de.      Legons   de  Geographic   Physique,   p.  263,  Paris, 

1898. 

52.  Kayser,  Emanuel.     Lehrbuch  der  Geologie.     I,  Pt.  4,  493,  Stuttgart, 

1912. 

53.  Scott,  W.  B.     An  Introduction  to  Geology,  p.  171,  New  York,  1911. 

54.  Hahn,   F.   G.     Untersuchungen  liber   das   Aufsteigen  und  Sinken  der 

Kiisten,  p.  23,  Leipzig,  1879. 

55.  Haage,  Reinholde.     Die  Deutsche  Nordseekliste,  p.  26,  Leipzig,  1899. 

56.  Ramsay,  A.  C.     On  the  Denudation  of  South  Wales  and  the  Adjacent 

Counties  of  England.     Mem.  Geol.  Surv.  Great  Britain.     I,  327,  1846. 

57.  Green,  A.  H.     Physical  Geology.     2nd  Edition,  p.  414,  London,  1877, 

58.  Jukes-Brow'ne,  A.  J.     Handbook   of  Physical  Geology,   pp.  182-183, 

London,  1892. 

59.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  702,  Boston,  1909. 

60.  Gulliver,  F.  P      ShoreUne  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  175,  1899. 

61.  Fenneman,  N.  M.     Development  of  the  Profile  of  Equilibrium  of  the 

Subaqueous  Shore  Terrace.     Jour,  of  Geol.     X,.  1-32,  1902. 

62.  Mitchell,  Henry.     On  the  Reclamation  of  Tide-lands  and  its  Relation 

to  Navigation.     Report  of  U.  S.  Coast  Survey  for  1869.     Appendix  5, 
pp.  85-86,  1872. 


REFERENCES  271 

63.  Stevenson,  Robert.     On  the  Bed  of  the  German  Ocean  or  North  Sea. 

Memoirs  Wermerian  Nat.  Hi.st.  Soc.  Trans.     Ill,  327,  1821. 

64.  Barrell,  Joseph.     Criteria  for  the  Recognition  of  Ancient  Delta  De- 

posits.    Bull.  Amer.  Geol.  Soc.  XXIII,  403-411,  1912. 

65.  Barrell,  Joseph.     Relative  Geological  Importance  of  Continental,  Lit- 

toral, and  Marine  Sedimentation.     Jour  of  Geol.     XIV,  318-319,  447- 
449,  1906. 

66.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

■p.  285,  Bo.ston,  1909. 

67.  Ibid.,  pp.  180-181. 

68.  Davis,  W.  M.     Die  Erklarende  Beschreibung  der  Landformen.     565  pp., 

Leipzig  and  BerUn,  1912. 

69.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  181,  Boston,  1909. 

70.  Johnson,  Douglas  W.     Youth,  Maturity  and  Old  Age  of  Topographic 

Forms.     BuU.  Amer.  Geog.  Soc.     XXXVII,  649-650,  1905. 

71.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  272,  Boston,  1909. 

72.  Davis,  W.  M.     Die  Erklarende  Beschreibung  der  Landformen,  p.  514, 

Leipzig  and  Berlin,  1912. 

73.  Geike,  A.     Textbook  of  Geology.     4th  Edition.     I,  593,  London,  1903. 

74.  Ibid.,  p.  590. 

75.  Williams,  H.  S.     Geologic  Biology,  pp.  62-63,  New  York,  1895. 

76.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  291,  Boston,  1909. 

77.  Ibid.,  p.  341. 

78.  Ibid.,  p.  346. 

79.  Ibid.,  pp.  340,  344. 

80.  Ibid.,  180,  272-276,  285,  289. 

81 .  Gilbert,  G.  K.     Lake  Bonneville.     U.  S.  Geol.  Surv.  Mon.     I,  40,  1890. 

82.  Beaumont,    Elie     de.      Lemons   de   Geologic    Pratique,    pp.    223-252, 

Paris,  1845. 
83.,   Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 
p.  708,  Boston,  1909. 

84.  Gilbert,  G.  K.     Lake  Bonneville.     U.  S.  Geol.  Surv.  Mon.     I,  40,  1890. 

85.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  710,  Boston,  1909. 

86.  Shaler,  N.  S.     PreUminary  Report  on  Sea-Coast  Swamps  of  the  Eastern 

United  States.     U.  S.  Geol.  Surv.  6th  Ann.  Rept.,  p.  364,  1886. 

87.  Barrell,  Joseph.     Criteria  for  the  Recognition  of  Ancient  Delta  De- 

posits.   BuU.  Amer.  Geol.  Soc.    XXIII,  385,  1912. 


CHAPTER  VI 
DEVELOPMENT  OF  THE  SHORELINE 

A.    SHORELINES    OF    SUBMERGENCE 

Advance  Summary.  —  It  is  the  purpose  of  the  present  chapter 
to  trace  the  systematic  development  of  the  shorehne  of  submer- 
gence from  its  initial  stage  of  extreme  irregularity  and  complexit}'- 
until  it  acquires  the  regular  and  simple  outline  characteristic  of 
full  maturity.  The  features  of  old  age  are  reserved  for  attention 
in  a  later  chapter.  Special  consideration  is  given  to  those  ele- 
ments of  shore  form  normally  associated  with  the  stages  of  youth 
and  maturity,  such  as  beaches,  spits,  bay  bars,  looped  and  flying 
bars,  tombolos,  cuspate  bars  and  forelands,  marsh  bars,  and  bay 
deltas.  The  question  as  to  whether  it  is  desirable  or  practicable 
to  recognize  young,  mature,  and  old  stages  of  development  of 
each  of  these  particular  forms  is  discussed,  as  is  also  the  ques- 
tion as  to  which  marine  forces  are  principally  concerned  in  their 
construction.  The  various  forms  discussed  are  illustrated  by 
ideal  diagrams  and  by  maps  of  examples  taken  from  nature. 

Initial  Stage.  —  As  was  early  hinted  by  de  la  Beche^,  and 
later  more  clearly  stated  by  Dana-,  when  a  land  mass  is  sub- 
merged the  sea  enters  the  main  river  valleys  and  their  lower 
tributaries  for  a  distance  which  depends  upon  the  depth  of 
submergence,  comes  to  rest  against  the  more  or  less  steep  slopes 
of  adjacent  hills  or  mountains,  and  overflows  the  lowest  cols  or 
passes  which  separate  outlying  hills  from  the  higher  main  ridges 
or  divides.  The  initial  stage  (Fig.  45)  of  the  typical  shoreline  of 
submergence  is  therefore  characterized  by  an  exceedingly  irreg- 
ular shoreline,  many  times  longer  than  a  straight  line  connecting 
two  points  on  the  shore;  by  numerous  branching  bays  or 
drowned  valleys  in  which  comparatively  deep  water  is  found  a 
short  distance  offshore;  by  many  peninsulas  projecting  out  to 
sea;  by  the  presence  of  numerous  islands;  and  by  an  irregular 
sea-bottom  whose  inequalities  represent  the  former  hills  and  val- 
leys of  the  land.     Portions  of  the  coast  of  Maine  and  of  the 

272 


INITIAL  STAGE  273 

Chesapeake  Bay  region  (Fig.  -46)  are  bordered  by  shorelines  of 
submergence,  but  Uttle  changed  from  the  initial  form,  outside  of 
which  submarine  hills  and  valleys  are  clearly  shown  by  soundings. 
The  great  variety  of  form  which  initial  shorelines  of  submer- 
gence may  possess    has   already   l^cen   suggested   by   the    clas- 


FiG.  45.  —  Shoreline  of  submergence,  initial  stage. 

sification  of  shorelines  outlined  in  Chapter  IV.  Such  variety 
is  inevitable,  as  will  readily  appear  when  we  consider  that 
everything  which  affects  the  shape  of  the  land  must  also  affect 
the  form  of  the  shoreline  produced  when  the  sea  surface  comes 
to  rest  against  the  land.  A  land  mass  may  have  a  great  variety 
of  structures,  which  will  be  reflected  in  the  shore  forms.  Those 
structures  may  be  subjected  to  several  different  erosive  processes, 
each  of  which  produces  surface  forms  peculiar  to  itself,  and  hence 
leaves  its  impress  upon  the  shoreline  of  submergence.  Each 
erosive  process  may  be  in  any  stage  of  its  cycle  when  submer- 
gence occurs,  and  the  resulting  shore  features  will  vary  widely 
with  the  different  stages  of  land  form  development.  The  stage 
of  shoreline  development  reached  at  any  given  moment  since 
submergence  will,  of  course,  profoundly  affect  the  characteris- 
tics of  the  shore.  ' 

It  is  essential,  therefore,  to  a  clear  conception  of  the  charac- 
teristic features  of  any  shoreline  that  the  description  take  ac- 
count of  the  structure  of  the  land  mass,  the  process  or  processes 


274 


DEVELOPMENT  OF  THE  SHORELINE 


oL    o 


YOUNG  STAGE 


275 


by  which  the  land  mass  has  been  eroded,  the  stage  of  land  mass 
dissection  reached  when  submergence  occurred,  and  the  stage 
of  shoreline  development  reached  since  submergence.  To  say 
that  "  the  coast  of  Dalmatia  represents  a  region  of  folded 
mountains,  maturely  dissected  into  longitudinal  ridges  and  val- 
leys by  normal  stream  erosion,  and  then  slightly  depressed  to 
form  a  shoreline  of  submergence  which  is  now  in  the  youthful 
stage  of  its  development,"  will  bring  to  the  hearer  who  is  familiar 
with  the  elementary  principles  of  shoreline  development  a  clearer 
mental  picture  of  the  essential  characteristics  of  that  shoreline 
than  could  a  much  longer  and  more  detailed  account  of  indi- 
vidual bays,  peninsulas,  islands,  and  other  local  features. 
Greater  definiteness  may  be  given  to  the  mental  picture  if  the  ex- 
planatory description  quoted  above  is  made  to  include  a  statement 


Fig.  47.  —  Early  youth  of  a  shoreline  of  submergence,  showing  crenulate 

shorehne. 


as  to  the  relief  and  texture  of  the  topography  produced  by 
stream  erosion;  for  the  coast  will  be  bold  or  subdued  according 
as  the  relief  is  high  or  low,  and  the  bays  will  branch  moder- 
ately or  intricately  according  as  the  texture  is  coarse  or  fine. 

Young  Stage.  —  As  rapidly  as  submergence  brings  the  hill 
and  valley  slopes  within  reach  of  the  sea,  waves  attack  those 
slopes.  We  have  already  seen  that  in  the  early  stages  of  waA/e 
attack  the  cliff  profile  is  more  irregular  than  in  the  initial  stage, 


Bolus  Head 


Fig.  48.  —  Crenulate  shoreline  of  the  southwest  coast  of  Ireland. 

276 


YOUNG  STAGE 


277 


278 


DEVELOPMENT  OF  THE  SHORELINE 


M.Aha-caku 


because  resistant  and  non-resistant  rocks  are  unequally  affected 
by  wave  erosion.  In  a  similar  manner  the  initial  shoreline  is 
rapidly  made  extremely  irregular,  on  a  small  scale,  wherever 
the  land  presents  to  the  sea  rocks  of  unequal  resistance.  The 
hills  and  valleys  of  the  land  may  have  been  well  graded  and 
characterized  by  smooth,  flowing  contours,  in  which  case  the 
initial   shoreline   must    be   composed    of    well-rounded    curves. 

But  early  in  the  youth 
of  the  shoreline  the 
curves  will  be  changed 
to  sharply  and  irregu- 
larly crenulate  lines 
by  differential  wave 
erosion-^  (Fig.  47).  In 
other  words,  although 
the  ultimate  goal  of 
wave  erosion  is  to 
make  a  shoreline  of 
submergence  less 
irregular,  as  will  pres- 
ently appear,  the  first 
effect  is  to  make  it 
minutely  more  irreg- 
ular. We  may  call  a 
shoreline  of  this  char- 
acter a  crenulate  shore- 
line. The  shoreline 
of  southwestern  Ire- 
land (Fig.  48),  border- 
ing the  beautifully 
graded  hills  of  a  ma- 
turely dissected  and 
partially  submerged  land  area,  is  in  this  crenulate  stage  of  de- 
velopment, as  may  readily  be  observed  from  the  deck  of  a 
transatlantic  steamer  passing  near  the  coast  on  its  way  to 
Liverpool.  Portions  of  the  coast  of  Japan  (Fig.  49)  Ukewise 
afford  excellent  examples  of  crenulate  shorelines. 

During  early  youth  some  of  the  most  picturesque  features  of 
cliff  detail  begin  to  appear.  On  rocky  shores  isolated  pinnacles 
of  resistant  material  are  left  standing  for  a  time   in   front   of 


hau-caku 


Fig.  49.  —  Yomig  shoreline  of  submergence  near 
Idzuhara,  Japan,  showing  crenulate  stage. 
(From  Russian  map  based  on  Japanese  data.) 


YOUNG  STAGE 


279 


280 


DEVELOPMENT   OF  THE  SHORELINE 

Plate  XXXVI. 


Fingal's  Cave  on  the  island  ot  Staffa,  Scotland.     A  sea  cave  formed  by 
wave  erosion  in  columnar  basalt. 


YOUNG  STAGE  281 

the  main  cliff.  These  chimneys  or  stacks  (Plates  XXXIV  and 
XXXV)  may  be  sculptured  by  the  waves  into  very  striking 
forms.  Weaker  zones  are  exca^^ated  by  the  waves  into  sea  caves 
(Plates  XXXVI  and  XXXVII),  of  which  Fingal's  Cave  on  the 
island  of  Staff  a  is  a  well-known  example.  Where  a  projecting 
belt  of  rock  is  completely  pierced  by  the  wave  attack,  an  arch 
(Plate  XXXVIII)  is  formed.  In  front  of  the  cliff  low  tide  may 
expose  a  bare  rock  platform  representing  the  landward  edge  of 
the  marine  bench  upon  which  the  occasional  stacks  are  situated 
(Plates  XXII  and  XXVIII).  From  the  face  of  the  cliff  numer- 
ous landslides  (Plate  XXXIX),  usually  small  but  sometimes  of 
grand  dimensions,  are  precipitated  into  the  water  or  upon  the 
rock  platform  as  a  consequence  of  the  rapid  encroachment  of 
the  waves  along  the  cliff  base.  It  should  be  noted  that  while 
the  above-named  features  begin  to  appear  in  the  early  youth  of 
the  shoreline  of  submergence,  and  reach  their  most  abundant 
development  before  maturity  is  attained,  they  may  also  be  present 
on  fully  mature  shores. 

Because  of  wave  refraction,  the  seaward  ends  or  "headlands  " 
(Plate  XXXIII)  of  peninsulas  and  islands  are  more  vigorously 
attacked  than  other  parts  of  the  shore,  while  the  inner  ends  of 
the  bays,  or  "  bay  heads,"  suffer  least.  In  a  comparatively 
short  time,  therefore,  there  are  developed  cliffed  headlands  of 
striking  aspect  (Fig.  50,  ch) .  Part  of  the  material  eroded  from 
the  headlands  is  deposited  in  the  depressions  of  the  irregular 
seafloor,  a  second  part  is  carried  out  to  the  deep  sea,  while  a 
third  part  is  temporarily  built  into  various  types  of  beaches 
and  embankments. 

The  great  variety  of  forms  assumed  by  these  beaches  and 
embankments  is  dependent  upon  the  unorganized  condition*  of 
the  longshore  currents  near  a  young  shoreline  of  submergence, 
and  distinguishes  the  latter  from  all  other  classes  and  stages  of 
shorelines,  which  are  much  more  simple.  As  shown  by  Figure 
51,  tidal  currents  are  broken  up  and  deflected  in  various  direc- 
tions by  the  sinuosities  of  peninsulas,  islands,  and  drowned 
valleys,  whenever  they  impinge  upon  an  irregular  coast.  Beach 
drifting  under  the  influence  of  the  swell  and  of  direct  wind  waves 
will  be  equally  irregular,  and  will  often  be  opposed  to  the  direc- 
tion of  tidal  currents.  The  complexity  will  be  increased  wherever 
other  types  of  currents  are  disintegrated  against  the  irregular 


282 


DEVELOPMENT  OF  THE  SHORELINE 


YOUNG  STAGE  283 

shore.  Eddy  currents  are  unusually  numerous  along  such  a 
coast.  Wave-eroded  debris  which  is  moved  by  any  of  these 
currents  must  accordingly  be  built  into  an  almost  endless  variety 
of  isolated  forms  not  intimately  related  to  each  other. 

Beaches.  —  In  early  youth  no  very  extensive  beach  is  apt  to 
form  at  the  base  of  the  headland  cliffs,  although  narrow  headland 
beaches  (Fig.  50,  hh)  maybe  found  in  favored  localities,  especially 
if  the  cliff  is  composed  of  non-resistant  sand  or  other  material 


Fig.  50.  —  Young  shoreline  of  submergence,  showing  types  of  beaches, 
bars,  spits,  and  forelands. 

which  readily  disintegrates.  Most  of  the  debris,  however,  is 
swept  from  the  marine  bench  at  the  base  of  the  exposed  cliff 
as  rapidly  as  erosion  and  weathering  remove  it  from  the  cliff 
face.  Beach  drifting,  possibly  aided  by  other  types  of  longshore 
current  action,  propel  a  considerable  portion  of  the  debris  along 
the  shores  of  the  bays  toward  the  bay  heads.  These  latter  areas 
are  loci  of  deposition  because  wave  refraction  has  here  reduced 
wave  erosion  to  a  minimum,  because  beach  drifting  due  to  on- 
shore wind  waves  and  to  the  swell  is  far  more  potent  than  any 
beach  drifting  which  can  result  from  offshore  winds,  because  direct 
wind  currents  moving  from  the  ocean  surface  into  the  bays  are 
more  effective  than  wind  currents  originating  at  the  heads  of  the 
bays  and  moving  seaward,  and  because  flood-tide  currents  fol- 
lowing the  shores  of  a  narrowing  bay  are  apt  to  be  more  power- 


284 


DEVELOPMENT  OF  THE  SHORELINE 


> 
X 
X 
X 

< 


YOUNG  STAGE 


285 


ful  than  the  opposing  ebb  currents.  It  happens,  therefore,  that 
much  of  the  debris  eroded  from  the  headlands  is  built  into  bay- 
head  beaches  (bh)  at  the  inner  ends  of  adjacent  bays  (Plate  XL). 
The  material  in  transit  along  the  sides  of  the  bay  may  form  bay- 
side  beaches  (bs)  which  when  fully  developed  connect  the  usually 
unimportant  headland  beaches  with  the  more  often  well-devel- 
oped bay-head  beaches. 

Embankments.  —  As  may  be  observed  from  Figure  51,  the  shore 
currents  of  a  young  shoreline  of  submergence  sometimes  pass 


Fig.  51.  —  Initial  unorganized  condition  of  currents  along  a  young  shoreline 
of  submergence  (left  hand  figure)  compared  with  organized  condition 
which  obtains  when  the  stages  of  submaturity  or  maturity  are  reached 
(right  hand  figure).  Light  arrows  =  longshore  currents,  heavy  arrows 
=  offshore  currents. 


directly  across  the  mouths  of  subsidiary  bays  instead  of  closely  fol- 
lowing the  trend  of  the  shore;  or  an  offshore  current,  such  as  a 
planetary  or  large  eddy  current,  may  keep  its  course  past  the  outer 
headlands  but  little  influenced  by  the  bays.  Under  these  condi- 
tions the  shore  debris  may  be  built  out  into  the  water  in  the  form 
of  a  narrow  embankment  which  grows  by  an  excess  of  deposition 
at  its  seaward  terminus,  just  as  a  railroad  embankment  is  extended 


286 


DEVELOPMENT   OF  THE   SHORELINE 


I 


I— I 
X 

< 
Ph 


YOUNG  STAGE 


287 


by  the  dumping  of  car-loads  of  debris  at  its  free  end^.  In  the 
case  of  the  current-built  embankment,  deposition  takes  place 
partly  because  the  current,  which  is  comparatively  swift  where 
it  impinges  against  the  headland  or  the  already  completed  por- 
tion of  the  embankment  and  therefore  able  to  transport  a  large 
amount  of  debris,  loses  part  of  its  velocity  when  it  passes  into 
the  deeper,  open  water  off  the  bay  mouth;  and  partly  because 
the  debris,  as  soon  as  it  reaches  deeper  water,  is  no  longer 
effectively  agitated  by  normal  wave  action  which  in  shallow 
water  served  to  raise  it 
intermittently  into  the 
moving  water  of  the  cur- 
rent. The  seaward  side 
of  the  narrow  embank- 
ment is  acted  upon  by  the 
ocean  waves,  which  build 
its  crest  above  normal  sea- 
level  and  establish  a  pro- 
file of  equilibrium,  similar 
to  that  of  an  ordinary 
beach^  (Fig.  36).  The 
'qmet-water  side  may  have 
a  more  uniform  slope,  de- 
termined by  the  subaque- 
ous angle  of  repose  of  the 
deposited  material.  If 
the  debris  is  coarse,  the 
distal  end  of  the  embank- 
ment will  have  an  abrupt 
slope  to  deep  water,  which 
also  represents  the  subaqueous  angle  of  repose  of  the  material 
composing  the  embankment;  but  if  the  debris  is  fine,  deposition 
will  be  less  sudden,  and  the  distal  end  will  slope  more  gradually 
into  deep  water. 

Spits.  —  So  long  as  an  embankment  has  its  distal  end  termi- 
nating in  open  water,  it  is  called  spit  (Fig.  50,  s.  See  also  Figs. 
52  and  53) .  When  the  spit  first  begins  to  develop  the  longshore 
current  responsible  for  it  is  normally  so  effective  in  comparison 
with  other  currents  that  the  latter  have  little  or  no  effect  upon 
its  form.     Tidal  currents  may  pass  in  and  out  of  a  bay  at  right 


Fig.  .52.  —  Sand  spits  on  the  shore  of 
Port  Orchard,  Washington. 


288 


DEVELOPMENT  OF  THE  SHORELINE 


YOUNG   STAGE 


289 


angles  to  the  spit's  direction  of  advance,  or  beach  drifting  may, 
under  the  influence  of  onshore  winds,  tend  to  drive  the  debris  at 
the  terminus  into  the  bay;  but  deposition  by  the  dominant  long- 
shore current  lengthens  the  spit  so  rapidly  in  the  direction  of 
that  current's  intention  that  the  weak  or  intermittent  efforts  of 
contrary  currents  produce  no  sensible  effect.  With  the  continued 
growth  of  the  spit,  however, 
and  the  consequent  narrow- 
ing of  the  entrance  to  the  bay, 
tidal  currents  pass  the  end  of 
the  spit  with  an  ever-increas- 
ing velocity.  More  and  more 
of  the  debris  brought  by.  the 
longshore  current  is  carried  in 
toward  the  bay  by  the  flood 
tide,  thus  giving  a  landward 
deflection  to  the  embankment. 
Outflowing  currents  sweep 
some  of  the  debris  seaward, 
but  the  combined  effects  of 
the  longshore  current  and 
active  wave  erosion  normally 
prevent  any  marked  seaward 
deflection.  The  longshore 
current  may  itself  be  deflected 
toward  the  bay  by  the  flood 
tide,  thus  assisting  in  the  land- 
ward deflection  of  the  spit 
it  is  building.  Furthermore, 
when  the  spit  first  begins  to 
grow,  its  elongation  proceeds 
with  comparative  rapidity, 
because  the  water  is  shallow 


Fig.  53.  —  Simple  spit  (below)  and 
compound  recurved  spit  (above)  at 
entrance  to  Port  Moller,  Alaska. 


and  no  great  amount  of  debris  is  necessary  to  build  the  em- 
bankment up  to  the  surface.  tBufc  as  it  advances  into  deeper 
water,  more  and  more  of  the  debris  must  be  laid  down  in  the 
depths,  and  less  and  less  is  available  for  the  linear  extension  of 
the  spit.  Under  the  new  conditions  of  slow  advance  the  influ- 
ence of  flood  tide  and  of  beach  drifting  due  to  onshore  winds 
becomes  increasingly  apparent,   the  debris  at  the  terminus  is 


290  DEVELOPMENT  OF  THE  SHORELINE 

carried  farther  landward  before  new  supplies  are  laid  down  in 
front  of  it,  and  a  landward  deflection  of  the  spit  results.  Under 
these  and  other  similar  conditions  it  often  happens  that  the 
end  of  a  spit  is  more  or  less  strongly  curved  inward.  When 
the  growing  embankment  acquires  this  form  it  is  called  a  hooked 
spit,  or  better,  a  recurved  spit  (Fig.  50,  rs). 

The  forces  supplying  debris  to  the  longshore  current,  the 
longshore  current  itself,  and  the  contrary  currents  which  tend 
to  recurve  the  spit,  do  not  always  act  with  even  approximate 
uniformity.^  One  or  more  of  these  activities  may  have  a  very 
pronounced  intermittent  character.  In  such  a  case,  the  forces 
tending  to  elongate  the  spit  in  a  straight  or  slightly  curved 
line  may  prevail  for  a  period,  after  which  the  forces  operating 
to  recurve  the  spit  may  temporarily  gain  the  ascendancy.  The 
effect  of  this  intermittent  action  will  be  to  produce  a  spit  whose 
inner  side  is  diversified  by  a  series  of  landward  deflected  points 
representing  successive  recurved  termini.  To  this  interesting 
form  the  name  comipound  recurved  spit  (Fig.  50,  crs)  may  be 
applied. 

It  sometimes  happens  that  after  a  recurved  spit  is  formed, 
new  currents  arise  which  remove  material  eroded  from  the 
more  protected  parts  (usually  the  inner  side)  of  the  spit,  and 
build  it  into  a  new  embankment  which  is  really  essentially  in- 
dependent of  the  form  from  which  it  projects.  The  original 
and  secondary  spits  do  not  curve  or  merge  into  each  other; 
on  the  contrary  their  lines  of  growth  intersect  at  distinct  angles, 
indicating  their  independent  relationship.  The  secondary  spit 
is  no  more  an  integral  part  of  the  original  spit  than  the  latter 
is  an  integral  part  of  the  cliffed  headland  from  which  it  springs. 
Since  it  is  desirable  to  give  the  combined  spits  a  single  name  be- 
cause of  their  association  in  nature,  we  may  speak  of  the  grouped 
features  as  a  complex  spit  (Fig.  50,  cs).  Sandy  Hook  is  an 
excellent  example  of  a  compound  and  complex  recurved  spit. 
The  landward  curvature  of  successive  termini  is  clearly  indi- 
cated by  the  contours  on  the  Sandy  Hook  topographic  quad- 
rangle. But  the  southwardly  deflected  embankments  which 
have  generally  been  regarded  as  representing  merely  an  extreme 
amount  of  recurving  of  the  original  spit  are  seen  on  closer 
examination  to  be  independent  secondary  spits  built  by  the 
waves  and  currents  of  Sandy  Hook  Bay  with  material  eroded 


YOUNG   STAGE 


291 


from  the  northwesterly  trending  embankments  of  the  original 
form  (Fig.  57). 

Occasionally  it  happens  that  variable  or  periodically  shifting 
cm'rents  extend  a  spit  first  in  one  direction,  then  in  another, 
giving  to  it  a  more  or  less  serpentine  pattern.  To  this  compara- 
tively rare  form  the  name  serpentine  spit  may  be  applied.  Gul- 
liver^ mentions  two  examples  of  this  type  in  his  essay  "^on 
"  Shoreline  Topography." 

As  the  cliff  from  which  a  spit  springs  is  cut  back  by  the  waves, 


■■^liliiiiii^^ 


\ 


Fig.  54.  —  Successive  stages  in  the  development  of  one  type  of  compound 

recurved  spit. 

the  spit  itself  is  driven  landward  at  the  same  rate.  This  retreat 
of  the  shore  leads  to  several  interesting  results,  as  will  readily 
appear  from  Figure  54.  If  aa'  is  the  original  position  of  the  spit, 
and  the  cutting  back  of  the  cliff  causes  the  point  of  its  tangency 
with  the  mainland  to  migrate  toward  the  left,  then  the  spit  will 
assume  the  several  positions  indicated  until  it  arrives  at  66'. 
As  will  be  observed,  the  spit  may  thus  acquire  a  compound 
form  with  a  succession  of  recurved  points  on  its  inner  side, 
without  necessarily  experiencing  any  extension  of  its  absolute 


292  DE\^LOPMENT  OF  THE   SHORELI>sE 

length.  It  should  l^e  noted,  furthermore,  that  the  progressive 
increase  in  the  length  of  the  ancient' points  does  not  in  this  case 
indicate  a  progressive  increase  in  the  relative  strength  of  the 
landward  moving  currents,  as  has  been  generally  assumed  in 
such  cases. 

The  marked  angle  at  which  the  present  shore  intersects  the 
axes  of  the  ancient  recurved  points  is  evidence  of  their  genetic 
relation  to  a  former  seaward  position  of  the  spit.  This  angle  is 
normally  greatest  in  the  case  of  the  oldest  points,  and  decreases 
progressively  in  those  which  are  of  more  recent  date;  but  increas- 
ing effectiveness  of  landward  moving  currents  may  sometimes 
cause  the  latest  points  to  bend  landward  at  increasingly  greater 
angles.  Should  the  spit  increase  in  length  at  the  same  time  that 
it  is  pushed  back,  we  may  have  a  case  in  which  the  compound 
feature  is  observable  onl}^  in  the  distal  portion,  the  landward 
end  being  a  simple,  straight  embankment  (Fig.  55).  This  does 
not  mean  that  the  two  unlike  parts  of  the  spit  have  had  different 
histories,  but  merely  that  the  landward  end  has  been  pushed 
completely  back  of  the  termini  of  the  recurved  points  which  for- 
merly existed  seaward  of  it. 

The  characteristics  of  a  retreating  compouip.d  spit  are  well 
seen  in  the  embankment  which  encloses  the  harbor  of  Toronto 
on  Lake  Ontario.  Figure  55,  based  on  charts  by  Hind,^  ex- 
hibits a  compound  distal  portion,  where  an  admirable  series  of 
recurved  points  are  separated  by  subparallel  ponds  or  lagoons, 
and  a  simple  landward  portion  which  has  been  pushed  back 
beyond  the  position  of  the  corresponding  points  in  that  region. 
The  greater  length  of  the  remaining  portions  of  the  more  recently 
formed  points,  and  the  greater  angle  which  the  oldest  points 
make  with  the  present  shore,  are  well  shown.  Judging  from 
the  charts  the  ends  of  the  recurved  points  are  truncated  by 
subordinate  spits,  which  give  the  whole  a  more  complex  form 
than  it  would  otherwise  have.  In  1854  Sanford  Fleming^  pub- 
lished an  excellent  essay  on  the  history  of  this  interesting  shore 
form,  clearly  setting  forth  the  essential  stages  of  its  development 
previous  to  the  time  of  his  study,  and  predicting  probable 
future  changes. 

In  the  case  just  mentioned  both  the  mainland  cHff  and  the  spit 
have  retreated  landward.  There  are  cases  in  which  the  cliff 
beyond  the  base  of  the  spit  is  cut  back  much  more  rapidly 


Page  293 


294 


DEVELOPMENT  OF  THE  SHORELINE 


than  at  the  point  of  attachment,  while  the  spit  as  a  whole  ad- 
vances seaward  instead  of  retreating.  Thus,  in  Figure  56,  where 
a  former  spit  (E)  springs  from  the  cliff  base  at  the  point  5,.  the 
cliff  southeast  of  this  point  is  cut  back  very  rapidly,  while  to  the 
northwest  there  is  little  or  no  cliff  erosion  and  the  mainland  is 
being  protected  by  the  growing  spit.     The  cutting  back  of  the 


r2 

\  '^^\a\        t^ 

Fig.  56.  —  Development  of  the  Cape  Cod  shoreline  (after  Davis).  As  the 
shore  facing  the  Atlantic  Ocean  is  cut  back  toward  the  west  {A,  C,  D)  the 
Provincetown  sand  spit  grows  progressively  seaward  {E,  G,  J),  and  the 
fulcrum  point  near  B  shifts  from  F'-  to  F^. 


cliff  at  the  southeast  produces  two  important  results:  First,  the 
direction  of  the  shoreline  is  changed,  so  that  the  longshore  cur- 
rent responsible  for  the  spit  comes  from  a  more  southerly  direc- 
tion and  hence  tends  to  maintain  its  covu'se  more  toward  the 
north  after  it  passes  B;  in  the  second  place,  the  cutting  back  of 
the  shore  removes  the  many  irregularities  which  formerly  disin- 
tegrated the  currents  impinging  upon  them,  a  single  vigorous 
current  along  the  simple  shoreline  replaces  the  many  weak  ones 
which  flowed  here  and  there  along  the  complex  shoreline,  and 
this  more  vigorous  current  will  maintain  its  more  northerly  course 
into  open  water  after  passing  the  point  B  because  there  is  no 


YOUNG  STAGE  295 

force  competent  to  deflect  it  into  the  bay  as  readily  as  the  origi- 
nal weaker  currents  were  deflected.  As  a  result  of  these  condi- 
tions, the  axis  of  spit-building  moves  progressively  seaward,  as 
the  cliff  to  the  south  is  pushed  progressively  landward.  Near 
the  point  B  there  is  a  "fulcrum"  {F),  north  of  which  the  shore- 
line is  everywhere  prograded,  while  south  of  it  there  is  only 
retrograding.  As  Davis  has  shown  in  his  classic  essay  on 
"  The  Outline  of  Cape  Cod,"^  which  contains  the  first  adequate 
presentation  of  the  fulcrum  idea,  the  fact  that  some  erosion  is 
experienced  at  the  point  B  and  on  the  adjoining  base  of  the  spit 
causes  the  fulcrum  point  to  shift  slightly  in  the  direction  of 
the  spit  from  F^  to  F^  in  Fig.  56.  (See  also  Fig.  57).  The 
Provincelands  of  Cape  Cod  and  Sandy  Hook  are  both  good 
examples  of  spits  formed  in  the  manner  above  described.  In 
the  case  of  Sandy  Hook  the  position  of  the  earliest  part  of  the 
spit,  corresponding  to  E  of  Figure  56,  may  be  indicated  by  the  low 
sandy  beach  plain  on  the  northeast  side  of  Navesink  Highlands, 
while  Island  Beach  represents  the  remnant  of  a  later  addition 
to  the  spit.  Sandy  Hook  itself  advanced  to  the  north  and  east 
by  the  successive  additions  of  recurved  points,  as  the  shore  near 
Long  Branch  was  driven  back  toward  the  west.  The  fact  that 
the  base  of  Sandy  Hook  spit  connects  with  a  bay  bar  at  the 
present  time,  instead  of  with  the  cliff  on  the  east  end  of  the 
Highlands,  introduces  a  slight  complication. 

Johnson  and  Reed  have  shown  that  in  so  complex  a 
series  of  spits  and  bars  as  that  composing  Nantasket  Beach 
on  the  Massachusetts  coast,  the  phenomenon  of  a  shifting 
fulcrum  between  a  retrograding  cliff  and  a  prograding  beach 
plain  may  occupy  an  important  place  in  the  history  of  the 
shoreline^". 

It  has  already  been  shown  that  the  distal  portion  of  a  spit, 
and  consequently  of  each  recurved  point  representing  a  former 
distal  portion,  is  submerged,  the  end  of  the  embankment  sloping 
down  into  deep  water  either  abruptly  or  gradually  according 
to  the  nature  of  the  debris  of  which  it  is  constructed.  The 
super-aqueous  portion  owes  its  height  primarily  to  the  waves, 
but  in  the  case  of  sand  spits  wind  action  may  locally  raise  the 
level  a  number  of  feet  by  forming  dunes.  Disregarding  the  dis- 
turbing effect  of  the  wind,  the  height  of  a  spit  will  depend  upon 
the  exposure  to  wave  action;    big  waves  will  cast  the  debris 


~I 


Fig.  57.  —  Development  of  Sandy  Hook  spit.  As  the  original  shore  between  Sea- 
bright  and  Long  Branch  was  cut  back  by  wave  attack,  the  zone  of  spit  forma- 
tion north  of  Navesink  Highlands  advanced  toward  the  northeast.  The  fulcrum 
point,  dividing  the  zone  of  retrograding  shoreline  from  that  of  prograding  shore- 
line, shifted  progressively  from  F^  to  F^.  West  of  the  letters  "oo"  in  "Hook" 
is  a  small  southward-pointing  spit  built  by  waves  from  the  northwest  out  of 
material  eroded  from  the  recurved  points  of  the  main  spit. 

Page  296 


YOUNG  STAGE  297 

many  feet  above  mean  water  level,  while  small  waves  will 
raise  the  surface  but  slightly  above  the  lake  or  sea.  Since 
the  exposure  of  a  recurved  spit  to  wave  action  is  unequal,  it 
"follows  that  some  portions  must  be  higher  than  others;  and  it 
is  normally  the  case  that  the  distal  curved  portion,  which  is 
acted  upon  bj-  the  smaller  waves  of  the  bay  into  which  it  is 
T)eing  deflected,  has  a  distinctly  lower  crest  line  than  the  rest 
of  the  spit.  The  importance  of  a  proper  appreciation  of  this 
simple  relation  will  appear  when  one  remembers  that  the  low 
altitude  of  the  crests  of  the  recurved  points  in  a  compound  spit 
have  erroneously  been  regarded  by  some  observers  as  a  proof  of 
coastal  subsidence. 

The  successive  embankments  added  to  a  growing  compound 
spit  may  be  closely  spaced,  with  shallow  depressions  between 
them  whose  bottoms  do  not  extend  as  low  as  sealevel;  or  the 
embankments  may  be  widely  spaced  and  separated  by  lagoons 
of  faii'ly  deep  water.  If  the  supply  of  debris,  longshore  current 
action,  and  the  activity  of  other  currents  are  fairly  uniform  and 
constant,  the  successive  embankments  will  be  closely  spaced 
and  tend  to  form  a  continuous  plain,  which  we  may  call  a  beach 
plain  in  view  of  the  fact  that  it  is  composed  of  ])each  deposits 
cast  up  by  the  waves.  It  very  seldom  happens  that  all  forces 
operating  at  the  shore  are  so  uniform  and  continuous  as  to  give 
a  perfectly  smooth  plain  surface;  on  the  contrary,  the  surface 
of  the  beach  plain  ordinarily  shows  a  series  of  low  ridges  repre- 
senting the  crests  of  beaches  built  by  the  waves  along  successive 
positions  of  the  shoreline.  These  heack  ridges,  or  "  fulls "  as 
the  English  geologists  call  them,  constitute  lines  of  growth  of 
the  beach  plain,  and  when  well  preserved  enable  one  to  trace 
the  history  of  development  with  great  accuracy.  They  vary  in 
altitude  according  to  exposure  to  wave  attack,  but  from  three 
to  twenty  feet  above  ordinary  high  water  level  may  be  taken 
as  the  more  common  elevations.  Beach  ridges  are  conspicuous 
features  of  certain  other  coastal  forms  besides  spits,  and  will  be 
further  considered  when  those  forms  are  described. 

If  any  one  or  more  of  the  forces  involved  in  spit  building 
operate  very  irregularly  or  intermittently,  it  may  happen  that 
successive  embankments  will  be  built  at  wide  intervals.  Let  us 
imagine  that  the  longshore  current  runs  much  more  swiftly  at 
rare  intervals  than  at  other  times.     For  a  long  period  it  may 


298  DEVELOPMENT  OF  THE   SHORELINE 

flow  too  slowly  to  remove  all  of  the  debris  eroded  from  the 
cliff,  and  a  large  beach  deposit  accumulates  at  the  cliff  base 
and  along  adjacent  parts  of  the  spit.  During  this  time  such 
material  as  the  current  does  transport  is  easily  carried  around 
the  recurved  point  and  in  toward  the  bay,  because  the  landward 
directed  currents  are  competent  either  to  move  the  load  of  a 
comparatively  weak  longshore  current  back  to  the  previously 
established  shoreline,  or  to  deflect  the  longshore  current  itself 
so  that  it  deposits  its  load  directly  along  that  shoreline.  Now 
let  us  suppose  that  the  longshore  current  is  accelerated.  Its 
increased  velocity  will  enable  it  to  pick  up  and  transport  the 
large  load  of  debris  which  accumulated  during  its  period  of 
sluggishness;  and  will  also  impel  the  current  to  maintain  its 
course  straight  ahead  into  deep  water,  instead  of  suffering  de- 
flection into  the  bay  when  it  reaches  the  recurved  point  of  the 
spit.  Furthermore,  the  shore  below  the  fulcrum  has  been  cut 
back  to  some  extent  during  the  period  since  the  last  recurved 
point  was  formed;  and  while  the  effect  of  this  was  not  readily 
apparent  so  long  as  the  longshore  current  was  comparatively 
sluggish,  a  vigorous  current  finds  at  once  that  the  prolongation 
of  its  normal  course  lies  to  seaward  of  the  distal  part  of  the 
spit.  Large  quantities  of  debris,  borne  by  a  current  which  de- 
parts from  the  fonner  shoreline  and  advances  into  open  water, 
must  be  built  into  an  embankment  which  elongates  rapidly  in  the 
direction  of  current  ^advance.  Waves  raise  the  surface  of  the  new 
embankment  into  a  beach  ridge;  and  by  repetitions  of  this  proc- 
ess there  are  formed  successive  beach  ridges  separated  by  lagoons 
of  considerable  breadth.  Irregular  or  intermittent  activity  of 
other  shore  processes  may  produce  the  same  result. 

Ordinarily  the  intermittent  character  of  shore  activities  is 
not  sufficiently  pronounced  to  cause  the  building  of  new  em- 
bankments so  far  removed  from  the  older  ones  as  to  have  really 
deep  water  between  them.  Usually  a  shallow  lagoon  or  merely 
a  marshy  swale  separates  the  ridges.  The  compound  recurved 
spit  at  Toronto  has  a  series  of  elongated  ponds  of  shallow  depth 
(Fig.  55),  as  has  also  the  compound  recurved  spit  known  as 
Presque  Isle  on  the  south  shore  of  Lake  Erie  (Fig.  58).  Sandy 
Hook  exhibits  close-set  ridges,  or  ridges  separated  by  shallow, 
marshy  swales.  It  is  possible  that  in  the  latter  spit  the  channels 
northeast  and  southwest  of  Island  Beach  (Fig.  57)  occupy  the 


YOUNG  STAGE 


299 


positions  of  lagoons  or  bays  between  former  ridges  now  largely 
destroyed. 

The  ultimate  length  of  a  spit  is  attained  when  tiie  tendency  ^ 
of  longshore  transportation  to  increase  that  length  is  just  bal- 
anced by  the  opposite  tendency  of  contrary  currents.     Where 


#1 
E       ,#^^^^^^ 

((ill 


Fig.  58.  —  Lagoons  and  ridges  of  the  Presque  Isle  compound  recurved  spit 

the  embankment  is  extending  across  a  bay,  progressive  length- 
ening narrows  the  inlet  through  which  tidal,  hydraulic,  and 
other  currents  must  pass  in  and  out,  and  thus  increases  the 
velocity  of  those  currents.  This  process  continues  until  the  veloc- 
ity of  the  latter  currents  is  just  high  enough  to  counteract  the 
constructive  tendency  of  the  longshore  currents,  when  the  em- 
bankment ceases  to  grow.  The  point  of  equilibrium  is  the  sooner 
reached  because  wave  action  on  the  seaward  side  of  the  embank- 
rnent  continually  reduces  the  size  of  the  particles  in  transit,  with 
the  result  that  the  farther  the  spit  advances  into  open  water 
the  less  powerful  are  the  cross  currents  required  to  remove 
material  from  its  distal  end.  Recurved  points  build  into  the 
bay  until  a  similar  condition  of  equilibrium  is  established  be- 


/j 


t 


300 


DEVELOPMENT  OF   THE   SHORELINE 


tween  the  currents  tending  to  lengthen  and  those  tending  to 
remove  the  point.  Because  of  the  varying  intensity  of  all 
shore  processes,  the  equilibrium  is  never  perfect,  but  only  approxi- 
mate;   and  the  end  of  the  embankment  therefore  advances  and 

retreats  intermittently  over  a  narrow 
zone  which  might  be  called  the  "zone 
of  equilibrium."  It  is  believed  by 
some  that  Sandy  Hook  has  reached 
this  zone  of  equilibrium,  and  that  the 
currents  into  and  out  of  New  York 
Bay  are  now  sufficiently  strong  to 
overcome  the  efforts  of  the  north- 
ward flowing  longshore  current  to 
increase  the  length  of  the  spit.  This 
view  is  expressed  by  Duane^^  in  the 
following  words:  "  It  (Sandy  Hook 
spit)  appears  to  have  reached  a  limit- 
ing length  at  which  the  currents  into  and  out  of  New  York  Bay 
have  sufficient  strength  to  scour  away  sand  deposited  at  its 
northern  end,  and  in  the  last  one  hundred  and  forty-five  years 
its  length  has  varied  only  about  2700  feet,  sometimes  increas- 
ing and  sometimes  decreasing." 

imj  Bars.  —  If  the  zone  of 
equilibrium  is  not  reached  by 
the  embankment  until  it  has 
almost  closed  the  inlet,  or  if 
the  longshore  currents  prevail 
throughout  and  succeed  in  ex- 
tending the  embankment  com- 
pletely across  the  bay,  the  spit 
becomes  a  bay  bar.  A  spit  may 
thus  change  to  a  bay  bar  inter- 
rupted by  a  narrow  inlet,  and 
this  in  turn  to  an  unbroken  bay 
bar.  Within  a  bay  converging 
currents  may  build  two  spits  toward  each  other  until  they 
form  a  bar  (Figs.  59  and  60).  As  a  rule  the  sea  tends  to  build 
bars  which  are  slightly  concave  toward  the  open  water;  or 
to  drive  back  the  central  part  of  a  bar  more  than  the  terminal 
portions  until  such  concavity  results.     But  where  an  embank- 


FiG.  60.  —  Spits  converging  to  form 
a  bay  bar  on  the  Alaskan  coast. 


YOUNG   STAGE 


301 


ment  grows  across  the  mouth  of  a  narrow  bay,  and  longshore 
current  action  is  very  powerful,  the  resulting  bar  may  be 
quite  straight. 

There  is,  however,  an  entirely  different  process  by  which  bars, 
indistinguishable  in  surface  form  from  those  developed  from 
growing  spits,  may  be  produced.  Waves  entering  shallowing 
water  may  break  before  reaching  the  coast,  and  cast  up  the 
bottom  debris  into  a  narrow  ridge,  in  the  manner  discussed 
more  fully  in  connection  with  "  Offshore  Bars."  The  irregular 
bottom  of  a  typical  young  shoreline  of  submergence  is  usually 
highly  unfavorable  to  this  process;  but  whenever  the  initial 
form  or  later  deposition  does  give  a  fairly  uniform  slope  to  the 


Fig.  61.  —  Bay-mouth  bars  on  the  Marthas  Vineyard  coast. 


bottom  near  the  shore,  wave  action  may  produce  a  bar  inde- 
pendently of  longshore  transportation.  Such  a  bar  may  form  a 
short  distance  offshore  and  be  driven  in  until  the  portion  oppo- 
site a  headland  becomes  a  headland  beach,  and  the  portion 
opposite  the  bay  remains  a  typical  bay  bar  extending  from 
headland  to  headland  and  nearly  or  quite  closing  the  bay  mouth; 
or  the  waves  may  construct  the  bar  just  at  the  mouth  of  the 
bay  in  the  first  place;  or  they  may  break  on  the  gently  sloping 
bottom  well  within  the  bay  and  produce  a  bar  near  the  middle 
or  even  near  the  head  of  the  bay.  It  is  possible  that  some 
supposed  sandspits  are  really  the  beginnings  of,  or  last  rem- 
nants of,  bars  formed  in  this  manner. 

A  compound  shoreline,  like  that  of  northern  New  Jersey,  is 


302 


DEVELOPMENT  OF  THE  SHORELINE 


especially  apt  to  have  an  offshore  bar  pushed  landward  against 
projecting  headlands,  after  which  it  will  appear  as  a  series  of 
shorter  bay  bars.  The  bar  across  the  mouths  of  the  Shrews- 
bury and  Navesink  Rivers  (Fig.  57)  may  have  had  an  earlier 
existence  as  an  offshore  bar  farther  out  in  the  Atlantic;  but  its 
history  is  not  altogether  simple,  for  it  has  been  temporarily 
breached,  and  later  rebuilt  in  part  at  least  by  longshore  currents. 
The  same  is  true  of  the  bay  bars  closing  Shark  River  and 
Manasquan  River,  which  probably  originated  as  parts  of  one 
offshore   bar;   while   Metedeconk   River   will   in   the  futm-e  be 

closed  by  the  northern  part  of 
the  offshore  bar  on  which  the 
town  of  Mantoloking  is  situated. 
It  does  not  seem  desirable  to 
give  separate  names  to  bay  bars 
formed  in  the  two  ways  above 
described,  because  of  the  fact 
that  the  method  of  origin  is 
often  obscure.  There  are  cases, 
of  course,  in  which  the  process 
of  formation  may  be  inferred 
with  reasonable  assurance  from 
the  form  or  position  of  the  bar; 
as  for  example  when  successive 
recurved  points  on  the  inner 
side  of  a  bar  indicate  its  development  from  a  compound  recurved 
spit,  or  the  abutting  of  the  bar  at  right  angles  against  the  shores 
of  the  bay  show  the  predominant  action  of  waves  breaking  on 
a  shelving  bottom.  It  is  probably  true,  however,  that  in  a 
majority  of  the  cases  where  offshore  wave  action  originates  a 
bay  bar,  longshore  transportation  plays  an  important  part  in 
its  further  development.  To  determine  the  relative  importance 
of  the  two  co-operating  forces  may  well  be  impossible.  We 
will  therefore  name  bay  bars  according  to  their  position  in  the 
bays  across  which  they  have  been  extended,  admitting  the  ex- 
istence of  two  processes  which  may  independently  or  in  co- 
operation produce  them.  On  this  basis  we  may  recognize  (1) 
hay-mouth  bars  (Fig.  50,  hmb)  or  those  extending  from  headland 
to  headland  across  the  mouths  of  bays,  excellent  examples  of 
which  are  found  along  the  shores  of  Marthas  Vineyard  Island 


Fig.  62.  —  Bay-mouth  bar  on  the 
Marthas  Vineyard  coast. 


YOUNG  STAGE 


303 


(Figs.  61  and  62) ;  (2)  hay-head  bars  (bhb)  or  those  built  a  short 
distance  out  from  the  shore  at  the  heads  of  bays,  like  the  outer 
bar  near  Duluth  at  the  head  of  the  westernmost  bay  of  Lake 
Superior  (Fig.  63);  and  (3)  mid-bay  bars  (mb)  or  those  built 
across  a  bay  at  some  point  between  its  mouth  and  its  head,  a 


Fig.  63.  —  Bay-head  bar  near  Duluth. 


good  example  being  the  bar  which  extends  nearly  across  the 
middle  of  Hempstead  Harbor,  Long  Island  (Fig.  64). 

A  headland  which  is  bordered  on  either  side  by  bay  bars 
or  spits  is  sometimes  called  a  winged  headland  ("  winged  be- 
headland  "  of  Gulliver^^)  Grassy  Hollow  headland  near  the 
eastern  end  of  Long  Island  is  a  typical  specimen  of  this  interest- 
ing form  (Fig.  65).  At  Long  Branch  on  the  New  Jersey  coast 
we  have  the  very  large  winged  headland  which  Gulliver  selected 
as  his  type  example. 

After  a  bay  bar  has  been  constructed,  the  pond  or  lagoon 
enclosed  behind  it  may  gradually  be  transformed  into  a  land 
area  through  the  combined  operation  of  several  agencies. 
Streams  from  the  land  bring  down  sediment  which  may  either 
be  distributed  over  the  floor  of  the  lagoon  by  current  action, 
or  built  into  a  bay  delta  which  advances  seaward  until  it  meets 


304 


DEVELOPMENT   OF  THE   SHORELINE 


the  bar.  Tidal  currents  carry  debris,  from  the  zone  of  wave 
agitation  outside  the  bar,  through  the  inlet,  and  distribute  it 
over  the  lagoon  bottom  or  build  it  into  a  tidal  delta  (Fig.  117) 
which  projects  into  the  lagoon  with  its  surface  usually  below 


Fig.  64.  —  Mid-bay  bar  in  Hempstead  Harbor,  Long  Island. 


sealevel.  Winds  from  the  sea  blow  sand  from  the  surface  of 
the  bar  into  the  lagoon  behind  it,  and  may  even  cause  sand 
dunes  to  migrate  some  distance  into  the  enclosed  area  of  quiet 
water.     Large   storm   waves   dash   over   the   crest   of   the   bar, 


YOUNG  STAGE 


305 


306 


DEVELOPMENT  OF  THE  SHORELINE 


and  their  waters  flowing  down  its  landward  side  build  wave 
deltas  (Plate  XLI)  into  the  edge  of  the  lagoon.  Salt  marsh  veg- 
etation may  secure  a  foothold  in  the  areas  of  shallower  water, 
and  both  by  building  up  to  the  surface  and  by  advancing  over 


BeacJ. 


Fig.  65.  —  Winged  headland  near  Sag  Harbor,  Long  Island. 

other  portions  of  the  lagoon  may  materially  hasten  the  conver- 
sion of  the  entire  area  into  land.  The  process  of  conversion 
goes  on  at  very  unequal  rates  in  different  places;  it  is  essentially 
independent  of  the  progress  of  shore  development  on  the  outer 
coast;  and  it  may  even  depend  mainly  on  forces  which  are 
not  directly  connected  with  marine  agencies. 

In  the  Uterature  on  shorelines  one  not  infrequently  encounters 
the  curious  idea  that  bay  bars  are  the  product  of  river  deposi- 
tion. The  material  of  the  bar  is  supposed  to  have  been  carried 
out  to  the  mouth  of  the  bay  and  dropped  where  the  brackish 
bay  water  meets  the  salt  water  of  the  sea.  Von  Richthofen" 
seems  thus  to  account  for  the  bay  bars  of  his  "  Liman  type  " 
of  coast,  the  standard  example  of  which  is  the  northwest  coast 
of  the  Black  Sea  near  Odessa;  and  this  theory  is  adopted  by 
HentzscheP^  and  others  in  discussing  the  same  and  similar  re- 
gions. How  the  coarse  sand  and  even  larger  debris  often  com- 
posing the  bar  could  be  carried  through  the  quiet  waters  of  the 


YOUNG  STAGE  307 

bay,  and  why  such  debris  was  not  deposited  in  the  form  of  a 
delta  where  the  river  enters  the  bay  head  are  matters  not  satis- 
factorily explained. 

Offsets,  Overlaps,  and  Stream  Deflection.  —  Where  a  bar  has 
almost  closed  a  bay  mouth,  a  narrow  tidal  inlet  maintains 
connection  with  the  open  ocean,  and  permits  tidal  currents  to 
pass  in  and  out  of  the  lagoon.  Rivers  emptying  into  the  lagoon 
may  increase  the  outflowing  currents;  and  where  there  is  no 
tide  the  opening  maintained  principally  by  the  outflow  of  river 
water  alone  might  better  be  called  an  outlet,  were  it  not  that 
similarity  of  form  and  the  desirability  of  uniformity  in  usage 
make  it  expedient  to  apply  the  single  term  "inlet"  to  all  these 
features. 

From  the  method  of  bar  development  it  follows  that  an  inlet 
is  normally  found  at  that  end  of  a  bar  toward  which  the  long- 
shore current  responsible  for  its  growth  is  moving.  It  frequently 
happens,  however,  that  storm  waves  break  through  a  bar  and 
establish  an  inlet  at  some  other  point,  often  at  or  near  the  point 
of  attachment  with  the  mainland  cliff.  Thereupon  the  original 
inlet  may  close,  while  the  new  one  begins  to  migrate  in  the 
direction  of  the  longshore  current  in  consequence  of  the  fact 
that  deposition  constantly  occurs  at  the  end  of  the  bar  on  the 
up-current  side  of  the  inlet,  necessitating  an  excess  of  erosion 
on  the  other  side  by  the  transverse  currents  which  insist  on 
keeping  the  inlet  wide  enough  to  permit  their  passage.  In  this 
manner  the  new  inlet  migrates  until  it  reaches  the  position  of 
the  original  inlet  at  the  down-current  end  of  the  bar,  when  the 
process  may  be  repeated.  Sometimes  the  older  inlet  is  closed 
before  a  new  one  is  opened,  and  the  bar  exists  for  some  time 
without  any  opening.  Shaler^^  was  of  the  opinion  that  new 
inlets  were  due  to  the  bursting  out  of  dammcd-up  land  waters 
which  had  been  held  in  restraint  by  an  unbroken  bar;  but  all 
the  evidence  available  seems  to  show  that  even  where  tidal 
influence  is  unimportant,  new  openings  are  most  frequently 
cut  from  the  seaward  side  by  the  attack  of  storm  waves.  On 
the  New  Jersey  coast  inlets  through  the  bars  which  obstruct  the 
mouths  of  Manasquan,  Shark,  and  other  rivers  or  bays  are  con- 
stantly closing  and  opening,  and  migrate  uniformly  in  the  direc- 
tion of  the  dominant  longshore  current. 

The  migrating  of  an  inlet  under  the  influence  of  a  longshore 


308 


DEVELOPMENT  OF  THE  SHORELINE 


current  is  commonly  accompanied  by  the  development  of  fea- 
tm-es  which  may  permit  one  to  determine  the  direction  of  the 
current  from  accurate  maps  or  charts.  In  many  cases  the 
part  of  the  bar  on  the  up-current  side  of  the  inlet  is  a  little 
farther  seaward  than  the  part  below  the  inlet,  in  which  case 
the  shore  is  said  to  be  offset  (Fig.  66,  a).  It  is  possible  to  have 
offsets  where  there  is  no  inlet,  as  shown  at  h  in  the  same  figure. 
Very  frequently  a  bar  which  offsets  its  remaining  portion  at  an 
inlet  also  overlaps  it  as  shown  at  c;  and  where  a  stream  enters 
the  sea  without  passing  through  a  bay,  the 
shifting  of  the  inlet  at  the  stream  mouth  may 
cause  a  pronounced  stream  deflection  (Fig.  66,  d). 
The  longshore  current  or  currents  responsible 
for  these  features  move  from  the  outer  toward 
the  inner  segments  of  the  shore,  or  in  the  direc- 
tion of  stream  deflection,  as  shown  in  the  figure. 
Direct  observation  of  shore  currents  is  compli- 
cated by  the  fact  that  at  the  time  of  observa- 
tion less  important  currents  in  an  opposite 
dh'ection  may  chance  to  prevail;  but  accord- 
ing to  Gulliver^^,  who  first  emphasized  the  im- 
portance of  offset,  overlap,  and  stream  deflection 
as  indicators  of  current  movements,  the  direc- 
tion of  the  dominant  current  is  reliably  indicated 
when  one  or  more  of  the  tlii'ee  features  just 
mentioned  is  present. 

There  is  reason  to  believe,  however,  that 
direct  wave  attack  may  force  one  segment  of 
a  bar  back  of  a  neighboring  segment,  thus 
giving  an  offset  which  is  quite  independent  of 
the  direction  of  longshore  currents.  It  might  well  happen  that 
the  resulting  offset  would  be  exactly  opposed  to  that  which  would 
have  been  produced  by  current  action.  This  seems  to  be  the 
case  on  the  southern  part  of  the  New  Jersey  shoreline,  where 
the  dominant  current,  as  shown  by  the  direction  of  inlet  migra- 
tion, is  southward;  yet  successive  offsets  give  a  false  indication 
of  a  northward  moving  current. 

Stone  Reefs.  — ■  Under  special  conditions  ordinary  ba}^  bars 
or  offshore  bars  may  undergo  a  peculiar  process  of  lithification 
which  changes  them  into  stone  reefs.      According  to  Branner^^, 


Fig.  66. 


YOUNG  STAGE  309 

who  has  described  the  remarkable  series  of  stone  reefs  border- 
ing the  coast  of  Brazil  for  a  distance  of   1250  miles  between 
Ceara  and  Porto  Seguro,  the  only  essential  difference  between 
these  reefs  and  ordinary  sand  bars  and  spits  lies  in  the  indura- 
tion of  the  upper  ten  or  twelve  feet  of  the  sand  through  the 
cementing   action   of   calcium   carbonate.     It   appears   that   in 
and  about  the  lagoons  or  ponds  back  of  the  bars  abundant 
aquatic    and    semi-aquatic    plants    live    and    die.     "  The    fresh 
water  is  thus  rendered  acid  by  the  presence  of  large  quantities 
of   carbon   dioxide   produced   by   organic   decomposition.     The 
acid  water  on  the  land  side  percolating  through  the  embank- 
ment of  sand  at  low  tide  attacks  the  calcareous  matter  (frag- 
ments of  shells,  etc.)  in  the  sand  and  passes  seaward  with  it  in 
solution,  but  as  it  comes  in  contact  with  the  dense  sea  water 
on  its  way  through  the  sand,  the  lime  carbonate  in  solution  is 
deposited  in  the  interstices  between  the  sand  grains.     In  time 
the  interstices  are  completely  filled,  and  the  sand  bank  is  hard- 
ened and  so  solidified  that  the  water  can  no  longer  soak  through 
it."     In  Branner's  opinion  the  essential  conditions  are  the  fol- 
lowing: lagoons  or  ponds  nearly  or  quite  closed  by  bars  or  spits; 
abundant  vegetation  in  or  about  these  water  bodies;   fragments 
of  shells,  crinoids,  coral,  or  other  calcareous  material  in  the  bar 
or  spit;    and  a  high  density  of  the  sea-water.     Stone  reefs  are 
rare  because  the  combination  of  all  these  features  is  rare.     Lithi- 
fied  beaches  originally  composed  of  sand  and  gravel  and  later 
cemented  by  calcium  carbonate  were  early  described  by  Beau- 
fort^^  from  the  coasts  of  Asia  Minor  and  Greece;    while  Cold^^ 
reports  stone  reefs  from  this  same  general  region  which  sepa- 
rate lagoons  from  the  open  sea  and  which  must  be  similar  to 
those  studied  by  Branner. 

Looped  Bars.  —  The  islands  of  a  young  shoreline  of  submer- 
gence are  attacked  from  all  sides  by  the  waves;  but  the  most 
effective  attack  is  delivered  upon  the  seaward  side,  because 
both  the  swells  and  the  largest  storm  waves  come  from  the 
open  sea,  and  because  wave  refraction  concentrates  the  energy 
of  both  types  of  waves  upon  the  seaward  side  of  islands  as  well 
as  upon  projecting  headlands.  As  in  the  case  of  headlands,  part 
of  the  eroded  debris  is  carried  out  to  a  permanent  resting  place 
in  deep  water,  part  is  temporarily  deposited  in  depressions  of  the 
irregular  sea  floor  near  the  land,  and  part  is  built  into  various 


310 


DEVELOPMENT  OF   THE  SHORELINE 


types  of  beaches  and  embankments.  Among  the  latter  there 
are,  in  addition  to  the  spits  and  bay  bars  already  described,  two 
forms  peculiar  to  eroded  islands:  looped  bars  and  tombolos. 

Little  beach  material  can  accumulate  at  the  base  of  the  ex- 
posed cliff  on  the  seaward  side  of  the  island.  Sometimes  spits 
extend  out  on  either  side  of  the  main  cliff,  usually  with  their 
axes  directed  backward  toward  the  land.  More  often,  perhaps, 
the  debris  is  driven  backward  along  either  side  of  the  island 
until  the  quieter  water  to  leeward  is  reached.  Here  embank- 
ments of  several  types  may  form.  Spits  may  trail  backward 
from  either  side,  maintaining  a  separate  existence;  or  their  ends 


Siiapka  I. 

700  Ft. 


Fig.  67.  —  Looped  bar  on  shore  of  Shapka  Island,  Alaska.     (C.  S.  Chart, 


may  unite  to  form  a  looped  bar  (Fig.  50,  lb).  Shapka  Island, 
Alaska  (Fig.  67),  and  Cup  Butte"-",  Utah,  furnish  good  examples  of 
looped  bars,  the  latter  existing  as  an  elevated  shore  feature  on  a 
former  shoreline  of  Lake  Bonneville.  In  other  cases  one  or  more 
embankments  will  be  extended  until  the  island  is  directly  con- 
nected with  the  mainland.  The  extension  of  the  embankment 
may  take  place  wholly  in  the  direction  from  the  island  toward 
the  mainland  (Fig.  68) ;  or  wholly  from  the  mainland  toward  the 
island,  especially  in  those  cases  where  longshore  currents  build 
a  spit  out  laterally  until  it  forms  a  bar  which  connects  with  an 
island  lying  to  one  side  of  the  cliffed  headland;  or  the  em- 
bankment may  be  constructed  from  both  directions  at  the  same 
time  until  the  ends  meet  to  form  a  connecting  bar;    or,  finally^, 


YOUXG  STAGE 


311 


the  bar  may  be  built  up  simultaneously  along  its  entire  length 
by  wave  action  on  a  shallowing  bottom  (Fig.  69).  Furthermore, 
the  mainland  may  be  replaced  in  any  of  the  above  instances  by 
another  island,  without  altering  the  essential  relations.  In  all 
of  these  cases  the  connecting  bar  is  called  a  tomholo  (Fig.  50,  t). 

Tombolos.  —  The  name  "  tombolo  "  was  applied  to  the  con- 
necting bar  by  Gulliver^i  in  the  following  words:  "  Upon  the 
coast  of  Italy  where  island-tying 
in  its  various  stages  is  beautifully 
shown,  such  a  bar  is  called  a  tom-- 
bolo.  For  convenience  in  distin- 
guishing   island-tying    bars    from 


Fig.  68.  —  Renard  Island  near  Seward, 
Alaska,  showing  embankment  grow- 
ing from  island  toward  mainland. 


Fig.  69.  —  Inner  Iliasik  Island, 
Alaska,  showing  embankment 
which  may  be  upbuilding  to- 
w'ard  the  surface  simultane- 
ously along  its  entire  length. 


those  of  other  kinds,  the  writer  proposes  to  call  every  bar  of  this 
kind  a  tomholo,  giving  an  English  plural  tombolos."  Professors 
Olinto  Marinelli  of  Florence  and  Giuseppe  Ricchieri  of  Milan  have 
both  expressed  to  me  orally  their  opinion  that  the  term  tombolo  in 
the  Italian  language  is  restricted  to  the  sand  dunes  found  upon 
shore  beaches  and  in  other  localities,  and  that  it  cannot  properly 
be  applied  to  a  bar  built  by  currents  and  waves.  There  seems 
to  be  no  doubt  that  the  plural  "  tomboli  "  does  signify  sand 


312 


DEVELOPMENT  OF  THE  SHORELINE 


dunes  or  similar  small  mounds.  On  the  other  hand,  it  would 
appear  that  failure  of  the  popular  mind  to  appreciate  the  inde- 
pendent origin  of  the  bars  and  the  dunes  which  surmount 
them,  had  resulted  in  the  application  of  the  term  tomboli  to 
the  bars  themselves,  at  least  in  some  parts  of  Italy.     This  fact, 


Fig.  70.  —  Single  tombolo  connecting  former  island  of  Marblehead  with  the 

mainland. 


and  the  confusion  of  ideas  responsible  for  it,  are  both  shown  in 
the  following  quotation  from  Pianigiani's  Dizionario  Etimolo- 
gico  delia  Lingua  Italiana^^:  "  '  Tomboli '  is  a  term  com- 
monly applied  figuratively  to  the  mounds  of  sands  which  the 
sea  forms  in  the  fashion  of  banks  on  the  shore;  otherwise  called 
'  cotoni  '  =  '  costoni  '  from  '  costa:  '  for  example,  '  the  sea, 
roughened  by  opposing  currents  or  winds,  scrapes  the  bottom 
and  brings  the  sand  back  to  the  shore,  forming  tumoli  or  tomboli, 
and  makes  bars  or  shoals  at  the  mouth  of  the  Arno.  These 
tomboli  are  the  same  thing  as  the  famous  dunes  of  the  Dutch 


YOUNG  STAGE 


313 


and  French'  (Targioni,  Viaggi)."*  Prof.  A.  A,  Livingston  of 
Columbia  University,  to  whom  I  am  indebted  for  calHng  my 
attention  to  the  foregoing  citation,  informs  me  that  small  mounds 


Fig.  71.  —  Former  island  of  Big  Nahant  tied  to  Little  Nahant,  and  the 
latter  to  the  mainland  by  single  tombolo. 


in  the  lagoon  at  Venice,  which  are  visible  only  at  low  water, 
are  called  "  tomboli  "  in  the  Venetian  dialect;  and  Prof.  F.  C. 
Ewart  of  Colgate  University  states  that  Petrocchi  gives  as  one 

*  "  '  Tomboli  '  si  chiamano  comunemente  per  similitudine  que'  monti- 
celli  di  rena,  che  il  mare  forma  a  guisa  d'argini  sulla  spiaggia,  altrimenti 
Cotoni  =  Costoni  da  Costa:  per  es,  "'  il  mare  tempestoso  per  traversia  rade  il 
fondo  e  riporta  al  lido  quella  rena,  e  forma  i  tumoli  o  i  tomboli,  e  fa  de' 
ridossi  o  interramenti  alia  bocca  d'Arno.  Essi  tomboli  sono  la  medesima  cosa 
che  le  famose  Dune  degli  Olandesi  e  Franzesi'.     (Targioni,  Viaggi.)  " 


314 


DEVELOPMENT  OF  THE  SHORELINE 


< 


YOUNG  STAGE  315 

meaning  of  the  word:  "  a  small  bank  of  sand  thrown  up  by  the 
sea."  On  maps  of  the  Instituto  Geografico  Militare  of  Italy 
such  names  as  "  Tombolo  della  Gianneha  "  and  "  Tombolo  di 
Feniglia  "  are  printed  along  bars  connecting  islands  with  the 
mainland.  The  fact  that  the  singular  form  ''tombolo"  is  used, 
rather  than  the  plural  "  tomboli  "  suggests  that  the  term  refers 
to  the  bar  itself,  and  not  to  the  series  of  dunes  which  may  occur 
upon  it. 

For  the  reasons  outlined  above,  and  for  the  further  reason 
that  the  term  tombolo  has  been  introduced  into  a  number  of 
English  discussions  of  shorelines,  and  even  into  some  reports 
published  in  foreign  languages,  it  seems  advisable  to  adopt 
Gulliver's  usage,  rather  than  to  use  the  double  term  "  connect- 
ing bar  "  (which  might  equally  well  apply  to  a  bay  bar  connect- 
ing two  headlands),  or  to  invent  a  new  term.  A  single  short 
term  is  desirable  for  the  form  under  discussion,  and  notwith- 
standing the  lack  of  uniformity  and  precision  in  the  Italian 
use  of  the  term  "  tombolo,"  its  adoption  into  the  English  lan- 
guage with  the  restricted  meaning  given  to  it  by  Gulliver  best 
meets  the  needs  of  the  case. 

If  the  former  island  is  connected  with  the  mainland  or  with 
another  island  by  a  single,  simple  bar,  we  have  a  single  tombolo 
(Fig.  70  and  Plate  XLII) .  On  the  Massachusetts  coast  Big  Nahant 
is  tied  to  Little  Nahant,  and  Little  Nahant  to  the  mainland  by 
single  tombolos  in  the  construction  of  which  onshore  wave  action 
on  a  shallowing  bottom  has  probably  played  an  important  part 
(Fig.  71).  A  beautiful  example  of  closely  similar  form  is  fur- 
nished by  Duxbury  Beach  and  Saquish  Neck  near  Plymouth 
Harbor  on  the  same  coast  (Fig.  72).  Islands  close  to  the  main- 
land, or  of  comparatively  large  extent  alongshore,  may  be 
connected  with  the  mainland  by  a  double  tombolo  or  even  a 
triple  tombolo.  Monte  Argentario  (Fig.  73)  is  tied  to  the  west 
coast  of  Italy  by  a  double  tombolo,  and  a  third  uncompleted 
bar  shows  that  the  connection  just  escaped  being  a  triple  tom- 
bolo. Where  two  embankments  extending  backward  from  an 
island  or  outward  from  the  mainland  unite  to  form  a  single 
ridge  before  the  connection  is  completed,  we  have  a  Y-tombolo, 
the  type  example  of  which  is  Morro  del  Puerto  Santo  (Fig.  74) 
on  the  Venezuelan  coast^*.  Complex  tombolos  result  when  sev- 
eral islands  are  united  with  each  other  and  with  the  mainland 


Gurnet  Pt. 


Fig.  72.  —  Duxbury  and  Saquish  Neck  tombolos  uniting  former  islands  with 
the  mainland  of  Massachusetts  near  Plymouth. 

Page  316 


YOUNG  STAGE 


317 


-,l,e0 


•jy 


Fig.  73.  —  Monte  Argentario,  Italy,  tied  to  the  mainland  by  a  double 

tombolo. 


318  DEVELOPMENT   OF   THE   SHORELINE 

by  a  complicated  series  of  bars.  Nantasket  Beach  (Figs.  75  and 
76)  on  the  Massachusetts  coast  is  an  excellent  example  of  this 
form,  in  which  the  prograding  of  some  of  the  bars  has  produced 
a  series  of  beach  ridges  extending  over  a  breadth  of  half  a  mile. 
The  complicated  history  of  this  remarkable  tombolo  has  been 
fully  discussed  by  Johnson  and  Reed-^.     It  should  be  noted  that 

the  term  tombolo  refers  to  the  con- 
necting l)ar  itself,  and  not  to  the  former 
island  as  has  been  assumed  Ijy  several, 
including  Hobbs'^,  who  employs  the 
speUing  ''  tombola  "  and  assigns  to  it 
a  Spanish  origin.  This  last  was  pre- 
sumably due  to  an  oversight,  as  I  can- 
not find  authority  for  a  Spanish  form 
of  the  word. 
Fig.  74.  —  Morro  del  Puerto  Cusvate  Bars.  —  It  occasionally  hap- 

Santo,  Venezuela,  a  Y-  j.i     ^  -j.      u-  i    i  ^  ^ 

,   ,  pens  that  a  spit  which  has  advanced 

tombolo  *^  ^    . 

some  distance  into  open  water,  re- 
curves (Fig.  77)  until  it  again  unites  with  the  shore  at  its  distal 
end,  thus  producing  a  bar  which  is  more  or  less  cuspate  according 
as  the  seaward  angle  is  fairly  sharp  or  broadly  rounded.  An  unu- 
sually sharp  angle  may  result  if  a  secondary  spit  trails  abruptly 
back  toward  the  shore  from  the  point  of  a  primary  spit.  In  other 
cases  two  spits  may  grow  out  from  the  shore  toward  each  other, 
and  finally  unite  to  form  a  bar  of  sharply  cuspate  form.  This 
often  happens  on  the  leeward  side  of  an  island,  when  it  represents 
the  early  stage  of  a  Y-tombolo.  Sometimes  the  presence  of  a 
shallow  some  distance  out  from  the  main  shore  will  cause  the 
development  of  a  bar  of  similar  form  whose  apex  is  at  the  shal- 
low. Essentially  identical  in  shape  and  origin  is  the  bar  which 
results  when  an  island  connected  with  the  mainland  by  a  double 
tombolo  is  consumed  by  the  waves,  leaving  a  V-shaped  bar 
with  the  point  of  the  V  near  the  site  of  the  former  island.  In 
both  of  the  last  two  cases  it  is  the  obstruction  in  front  of  the 
main  shore  which  determines  the  form  and  location  of  the 
resulting  bar. 

Certain  features  are  common  to  bars  developed  in  the  manner 
above  described.  All  of  them  are  more  or  less  cuspate  in  form; 
all  enclose  a  lagoon  or  swampy  area;  and  probably  all  have 
been  produced  by  the  combined  action  of  waves  and  currents. 


YOUNG  STAGE  319 

It  is  frequently  impossible  to  determine  the  precise  manner  in 
which  a  given  bar  originated  and  developed.  For  this  reason 
it  seems  wisest,  as  in  the  case  of  bay  bars,  to  group  the  similar 
forms  under  a  single  name,  recognizing  the  fact  that  different 
examples  may  have  originated  in  different  ways.  The  name 
V-bar  has  been  applied  to  some  of  these  forms;  but  because  of 
their  relation  to  cuspate  forelands,  described  below,  we  will 
employ  the  term  cuspate  bar  (Fig.  50,  cb). 

Were  the  compound  spit  (Fig.  55)  which  protects  Toronto 
Harbor  to  unite  with  the  shore  at  its  distal  end,  as  Fleming^^ 
considered  a  future  possibility,  we  would  have  a  compound  cus- 
pate bar.  Caseys  Point  (Fig.  78)  and  Gaspee  Point  (Fig.  79), 
Rhode  Island,  representing  what  Gulliver"  calls  the  V-bar  stage 
and  lagoon-marsh  stage  of  cuspate  forelands,  are  good  examples 
of  simple  cuspate  bars  wHich  were  probably  developed  from  spits 
growing  seaward  toward  each  other,  or  from  primary  spits 
growing  seaward  and  secondary  spits  extending  from  their  distal 
points  backward  to  the  shore.  At  the  southern  end  of  Revere 
Beach ,  near  Boston  is  a  cuspate  bar  produced  by  the  removal 
of  an  island  which  was  connected  with  the  mainland  by  a  double 
tombolo  (Fig.  80).  The  shores  of  Port  Discovery  on  the  Wash- 
ington coast  exhibit  a  l^eautiful  series  of  cuspate  bars  in  all  stages 
of  formation  (Fig.  81). 

Cuspate  Forelands.  —  In  none  of  the  shore  forms  thus  far 
considered  has  there  been  any  extensive  forward  building  of 
the  main  shore  into  the  water.  Beaches,  spits,  bay  bars,  tom- 
bolos,  and  cuspate  bars  are  either  comparatively  narrow,  or, 
as  in  the  case  of  some  broad  spits  and  tombolos,  are  connected 
with  the  land  by  narrow  emljankments.  We  must  now  con- 
sider a  group  of  forms  in  which  the  shoreline  is  systematically 
prograded  by  wave  and  current  action,  and  an  appreciable 
area  of  more  or  less  continuous  dry  land  added  to  that  pre- 
viously existing.  The  new  land  is  sometimes  called  a  beach 
plain;  or,  following  Gil])ert,  a  wave-built  terrace.  The  latter 
term  is  more  appropriate  for  the  examples  found  on  the  elevated 
shorelines  studied  by  Gilbert  than  for  those  on  modern  shores 
where  the  terrace  effect  is  not  evident  because  the  top  surface 
alone  appears  above  water.  We  will  follow  Gulliver's  sugges- 
tive terminology  and  speak  of  these  features  as  forelands.  They 
may  have  a  variety  of  forms,  but  where  most  typically  developed 


320 


DEVELOPMENT  OF  THE   SHORELINE 


LHI 


Sk 


■:■■■:■■:■  \kL 


:o^\BL 


f:.-;.;:--":;::-,  SL 


;••:::••.  WL 


Fig.  75.  —  Former  islands,  many  of  which  were  wholly  or  completely  eroded 
by  wave  action,  and  the  resulting  debris  used  by  the  waves  to  build  a 
complex  tombolo  tying  the  remaining  islands  to  the  mainland.  Dotted 
contours  show  islands  wholly  destroyed,  broken  contours  the  eroded 
portions  of  islands  but  partially  destroyed.     (Johnson  and  Reed.) 


YOUNG  STAGE 


321 


WP 


Hingham  Harbor 


Fig.  76.  —  Naiitasket  Beach,  Massachusetts,  the  complex  tombolo  formed 
by  wave  erosion  of  the  islands  shown  in  Fig.  75,  with  deposition  of  the 
debris  to  give  connecting  beaches  uniting  the  remaining  islands  with 
each  other  and  with  the  mainland  at  the  south.     (Johnson  and  Reed.) 


Pt.  Monroe 


Fig.  77.  — A  strongly  recurved  .si)it  on 
the  Washington  coast,  about  to 
become  a  cuspate  bar. 


k 

Gaspee  Point 

l^ 

M 

M 

Fig.  79.  —  Cuspate  bar  showing 
enclosed  marsh  near  Provi- 
dence, Rhode  Island. 


Boulders  (remains  or 

...■■■_    Cherry Jsland) 


Fig.  80.  —  Cuspate  bar  originally  built  as  a 
tombolo  tying  to  the  mainland  an  island 
since  removed  by  wave  erosion. 
Page  322  > 


^MTTeenesPL, 


5§^. 


jPlum 
S^Beach 


•CaseysPt. 


Fig.  78.  —  Cuspate  bars  on  the 
Narragansett  Bay  shore. 


Page  323 


324 


DEVELOPMENT  OF  THE  SHORELINE 


are  more  or  less  triangular  in  shape  with  the  apex  of  the  triangle 
pointing  out  into  the  water  (Fig.  82) ;  they  are  then  called  cus- 

pateforelands^^.  A  change  in 
the  outline  of  the  shore  or  in 
the  configuration  of  the  sea- 
bottom  often  occasions  their 
development,  while  in  other 
cases  no  assignable  cause  is 
apparent. 

There  is  no  sharp  dividing 
line  between  a  compound 
cuspate  bar  in  which  the 
successive  embankments  are 
closely  spaced,  and  a  cuspate 
foreland  in  which  the  differ- 
ent beach  ridges  are  widely 
enough  separated  to  enclose 
strips  of  lagoon  or  marsh.    Transition  forms  between  the  two  types 


Fig.  82.  —  Cuspate  foreland  near  Port 
Townsend,  Washington. 


Types  of  cuspate  foreland  bars. 


exist,  and  might  appropriately  be  termed  cuspate  foreland  bars 
(Fig.  83,  a).     Another  intermediate  form,  properly  classed  under 


YOUNG  STAGE  325 

the  same  term,  is  produced  when  a  typical  cuspate  bar  enclosing 
a  triangular  lagoon  or  marsh  (Fig.  83,  b)  is  prograded  by  the  addi- 
tion of  successive  beach  ridges  upon  its  seaward  side.  If  the 
lagoon  or  marsh  strips  are  a  minor  feature,  or  if  the  initial  tri- 
angular lagoon  is  very  small  as  compared  with  the  total  surface  of 
added  land,  then  the  forms  are  called  simply  cuspate  forelands. 

I  have  found  it  profitable  to  recognize  three  principal  types 
of  cuspate  forelands.  When  the  shore  is  aggraded  on  both 
sides,  so  that  fairly  symmetrical  lines  of  growth  (beach  ridges 
and  swales)  run  parallel  with  both  shores  of  the  cusp,  we  have  a 
simple  cuspate  foreland.  In  one  of  its  former  positions  Cape 
Canaveral  seems  to  have  been  a  fairly  good  example  of  this 
type  (Fig.  84).  Where  erosion  attacks  one  side  of  the  cusp 
to  such  an  extent  that  no  ridges  and  swales  remain  parallel  to 
that  shore,  but  the  shoreline  obliquely  truncates  these  lines  of 
growth,  a  truncated  cuspate  foreland  is  produced.  As  the  type 
example  of  this  form  we  might  cite  the  Darss  foreland  on.  the 
Baltic  coast  of  Germany  (Fig.  131),  whose  western  shore  abruptly 
truncates  a  magnificent  series  of  ridges  and  swales.  Occasion- 
ally a  truncated  cusp  of  this  type  is  later  prograded,  giving 
ridges  and  swales  parallel  to  the  new  shoreline;  and  the  proc- 
ess of  alternate  retrograding  and  prograding  may  be  repeated 
a  number  of  times  with  constantly  varying  direction.  The 
resulting  forms  will  be  designated  as  cojnplex  cuspate  forelands. 
To  this  class  belongs  the  present  Cape  Canaveral,  on  which 
several  distinct  series  of  ridges  and  swales  have  been  successively 
truncated  (Fig.  129).  The  Dungeness  (Fig.  130)  of  southeastern 
England  is  moderately  complex  near  its  seaward  point. 

It  should  be  noted  that  Cape  Canaveral  occurs  on  a  shoreline 
of  emergence.  It  is  cited  here  because  cuspate  forelands  occur 
on  all  classes  of  shores,  and  because  it  affords  unusually  good 
examples  of  two  of  the  three  types  of  cuspate  forelands  defined 
above. 

Marsh  Bars.  —  An  interesting  form,  not  generally  recognized, 
is  produced  by  marine  erosion  of  the  seaward  edge  of  a  marsh 
which  was  originally  unprotected  from  the  sea  by  any  barrier 
of  sand  or  gravel.  Wave  attack  separates  the  vegetable  matter 
of  the  marsh  from  the  sand  which  is  usually  present  in  greater 
or  less  amount,  and  casts  the  sand  upon  the  edge  of  the  re- 
maining marsh  in  the  form  of  a  narrow  ridge.     On  the  map  such 


Page  326 


YOUNG  STAGE 


327 


a  ridge  will  look  like  a  narrow  offshore  bar  with  a  later  formed 
marsh  back  of  it.  In  reahty  the  marsh  is  the  older,  and  the 
ridge  is  quite  unlike  an  offshore  bar  in  origin.  The  smaller 
size,  lack  of  contmuity,  and  the  irregular  pattern  of  these  marsh 
bars  will  generally  distinguish  them  from  true  offshore  bars. 
They  are  usually  found  bordering  unexposed  shores  where  un- 
protected marsh  deposits  could  persist  for  a  long  time,  suffer- 
mg  only  gradual   removal   by  small   sized    waves.     Along    the 


Fig.  85.  —  Marsh  bars  on  the  Delaware  Bay  shore. 

Delaware  Bay  shores  of  New  Jersey  marsh  bars  are  numerous, 
Robinson's  Beach  near  the  mouth  of  Dennis  Creek  being  a  good 
example  (Fig.  85).  The  fact  that  some  foreign  debris  may  be 
brought  to  such  a  bar  by  waves  and  currents  is  immaterial,  the 
essential  point  being  that  the  marsh  is  older  than  the  bar,  and 
has  never  been  bordered  by  a  true  offshore  bar  formed  by  wave 
action  on  the  sea-bottom. 

Flying  Bars.  — ■  Aiter  a  spit  or  looped  bar  has  grown  backward 
from  an  island,  it  sometimes  happens  that  the  island  itself  is 
entirely  removed  by  wave  attack  before  the  spit  or  bar  is  de- 


328  DEVELOPMENT  OF  THE  SHORELINE 

stroyed.  We  then  have  a  flying  bar,  isolated  in  the  open  water. 
GulUver-^,  who  originated  the  term,  suggested  that  Sable  Island, 
an  isolated  bar  of  unconsolidated  sand  off  the  coast  of  Nova 
Scotia,  may  be  a  flying  bar  left  in  its  exposed  position  by  the 
consumption  of  a  former  island  to  which  it  was  attached. 

Bay  Deltas.  —  Streams  entering  the  heads  of  drowned  valleys 
will  deposit  sediment  to  form  deltas,  providing  their  currents 
transport  more  debris  or  debris  of  larger  size  than  the  marine  cur- 
rents in  the  bay  can  remove.  The  deltas  normally  advance  from 
the  heads  of  the  bays  toward  the  bay  mouths,  and  may  be  des- 
ignated by  the  term  hay  deltas  (Fig.  50,  bd).  They  often  extin- 
guish the  lagoons  left  back  of  bay  bars,  or  completely  fill  open 
bays,  thus  assisting  the  shore  processes  in  their  efforts  to  sim- 
plify the  shoreline.  It  should  be  remembered,  however,  that 
they  are  the  products  of  normal  stream  action,  and  are  deposited 
in  spite  of  marine  processes,  rather  than  because  of  them.  For 
this  reason  it  is  a  mistake  to  treat  them  as  one  of  the  forms 
resulting  from  the  normal  tendency  of  marine  forces  to  simplify 
ragged  coast.  We  must  rather  regard  them  as  extraneous 
features  whose  effect  in  straightening  the  shoreline  is  wholly 
incidental  and  accidental,  and  quite  independent  of  the  processes 
by  which  waves  and  currents  work  toward  the  s^me  result. 

Stages  of  Development  of  Shore  Details.  —  It  may  have 
been  observed  that  in  the  preceding  discussions  of  beaches,  spits, 
bars,  tombolos,  forelands,  and  deltas,  no  account  has  been  taken 
of  successive  stages  of  development  of  these  forms.  They  have 
been  described  as  forms  especially  characteristic  of  a  young 
shoreline  of  submergence,  but  young,  mature,  and  old  stages 
of  recurved  spits,  bay  bars,  and  all  the  other  forms  mentioned, 
have  not  been  recognized.  The  omission  was  intentional,  and 
is  due  to  the  writer's  doubt  of  the  wisdom  of  attempting  to  clas- 
sify the  details  of  shore  forms  into  definite  stages  of  development. 
Inasmuch  as  this  doubt  has  not  been  shared  by  all  students  of 
shoreline  physiography,  it  is  desirable  that  the  grounds  for  its 
existence  be  made  plain. 

The  greatest  value  of  recognizing  sequential  stages  of  land- 
form  evolution  is  the  aid  thus  given  to  a  clear  comprehension 
of  the  shape  and  significance  of  the  forms  in  question.  Assuredly, 
the  introduction  of  the  evolutionary  idea  into  the  study  of 
river  valleys,  coastal  plains,  mountains,  and  other  major  land- 


STAGES  OF  DEVELOPMENT  OF  SHORE   DETAILS       329 

forms  has  shed  a  flood  of  hght  upon  their  present  shapes  and 
their  past  and  future  histories.  We  have  seen  that  the  shore 
profile  cannot  be  fully  understood  except  in  the  light  of  its 
successive  and  orderly  stages  of  development;  and  the  same  is 
true  of  the  outline  of  the  shore  as  a  whole.  On  the  other  hand, 
it  may  well  be  doubted  whether  it  is  profitable  to  push  the 
developmental  idea  so  far  as  to  apply  it  in  the  explanation  of 
all  the  detailed  forms  which  are  merely  incidents  in  the  evolu- 
tionary history  of  some  major  topographic  unit.  The  term 
"  young  river  "  is  full  of  significance  for  the  student  of  land- 
forms;  but  I  doubt  whether  anyone  will  profit  from  an  attempt 
to  recognize  young,  mature,  and  old  stages  of  sandbars,  which 
may  occur  in  any  or  all  of  the  different  stages  of  river  develop- 
ment. Similarly,  I  find  unlimited  value  in  the  recognition  of 
young,  mature,  and  old  stages  of  shorelines;  but  am  not  con- 
vinced that  there  is  profit  in  the  effort  to  classify  all  the  details 
of  a  young  shoreline,  for  example,  into  three  or  more  special 
stages  of  development.  Unless  it  shall  appear  that  the  under- 
standing of  shore  forms  is  materially  aided  by  such  attempted 
classification,  we  may  better  restrict  the  application  of  terms 
indicative  of  developmental  stages  to  the  shoreline  as  a  whole, 
rather  than  extend  their  use  to  each  of  its  many  parts. 

A  further  reason  for  not  recognizing  definite  successive  stages 
in  the  development  of  spits,  bars,  forelands,  etc.,  is  the  difficulty 
of  determining  any  regular  and  orderly  succession  of  features 
which  will  be  common  to  all  forms  of  a  given  class,  and  which 
are  genetically  related  to  true  shoreline  processes.  The  best 
attempt  to  classify  shore  details  into  stages  of  development  is 
that  made  by  Gulliver;  but  the  results  of  that  attempt  are 
not  altogether  satisfactory.  Thus  the  youth  of  a  tombolo  is 
assumed  to  be  represented  by  one  or  two  cuspate  forelands 
projecting  from  mainland  toward  island,  or  island  toward  main- 
land, or  both,  even  though  the  intervening  channel  may  be  so 
deep  that  further  growth  of  the  forelands  is  impossible.^"  On 
this  basis,  a  tombolo  which  had  been  entirely  completed,  and  then 
broken  through  by  storm  waves,  would  be  called  "  young." 
A  completed  tombolo  is  said  to  be  in  "  adolescence,"  or,  if  the 
island  happens  to  be  nearly  or  quite  eroded  away,  "  late  adoles- 
cence;" while  "the  mature  stage  of  island-tying  is  where  the 
islands  and  their  connecting  tombolos  are  completely  consumed 


1,'SO 


Fig.  47 


Fig.  50. 


STAGES  OF   DEVELOPMENT  OF   SHORE   DETAILS       331 


Fig.  88. 
Comparison  of  text  figures  to  facilitate  correlation  of  successive  stages  in 
the  development  of  a  shoreline  of  subm?rgence. 

Fig.  45.  —  Initial  stage. 
Fig.  47.  —  Early  youth. 
Fig.  50. —Youth. 
Fig.  87.  —  Submaturity. 
Fig.  88.  —  Maturity. 
hd,  bay  delta;    bh,  bayhead  beach;    bhb,  bayhead  bar;    bmb,  baymouth  bar; 
bs,  baj'side  beach;    cb,  cuspate  bar;    cf,  cuspate  foreland;     ch,   chffed 
headland;   crs,  compound  recurved  spit;   cs,  complex  spit;    hb,  headland 
beach;    lb,  looped  bar;     mb,  midbay  bar;   rs,  recurved  spit;    s,  spit;    t, 
tombolo;  wh,  winged  headland. 


332  DEVELOPMENT  OF  THE   SHORELINE 

by  the  sea''^"  Surely  the  developmental  idea  is  forced  beyond 
the  limits  of  its  usefulness  when  the  complete  annihilation  of  a 
given  form  is  called  its  "  maturity."  One  reason  for  the  unsatis- 
factory character  of  this  classification  is  that  it  represents  an 
attempt  to  harmonize  the  stages  of  tombolo  formation  with  the 
stages  of  shoreline  development,  an  attempt  which  must  always 
end  in  failure  for  the  reason  that  the  isolated,  detailed  forms  of 
an  irregular  shoreline,  even  if  they  develop  systematically,  cannot 
develop  synchronously  with  the  shoreline  as  a  whole.  Tombolos 
may  be  made  and  destroyed  while  the  shoreline  is  still  in  its 
youth. 

Gulliver's  attempt  to  classify  bay  bars  according  to  stages  of 
development  is  equally  unsatisfactory.  The  basis  of  classifi- 
cation was  made  the  extent  to  which  the  bay  was  filled  by  a 
stream  delta  or  other  deposits.  Since  these  deposits  are  quite 
independent  of  the  bar  itself,  and  are  found  abundantly  in  bays 
which  have  no  bars,  they  can  scarcely  be  accepted  as  a  proper 
basis  for  the  classification  of  bars  into  young,  adolescent,  and 
mature  examples.  If  bay  bars  have  any  orderly  sequence;  of 
forms  characteristic  of  different  stages  of  their  development, 
they  must  be  classified,  if  at  all,  on  the  basis  of  those  forms,  and 
not  on  the  relative  size  of  wholly  extraneous  features,  such  as 
river  deltas,  which  may  happen  to  lie  back  of  them. 

In  discussing  cuspate  forelands  Gulliver  drops  the  terms  youth, 
adolescence,  and  maturity,  for  reasons  which  are  not  clear,  and 
speaks  of  "  three  stages  of  progressive  development,  — the  V-bar 
stage,  the  lagoon-marsh  stage,  and  the  filled  stage."  He  recog- 
nizes that  the  first  two  stages  are  not  represented  in  the  history 
of  those  forelands  which  build  out  continuously  from  the 
mainland.  Bay  deltas  are  classified  as  young,  adolescent,  or 
mature  according  to  the  extent  to  which  they  fill  the  bay  into 
which  they  happen  to  be  built.  Of  three  deltas  identical  in  size, 
shape,  and  composition,  but  l^uilt  into  three  bays  of  increas- 
ing length  measured  from  head  to  mouth,  one  would  be  called 
mature,  another  adolescent,  and  the  third  young.  Ordinary 
deltas  are  classified,  not  according  to  stages  of  development, 
but  according  to  form  as  determined  by  the  ratio  of  activity 
between  river  and  marine  currents,  because  it  was  not  found 
practicable  to  discover  laws  of  progressive  delta  development 
when  the  deltas  did  not  occur  in  bays.     This  fact  must  lead  us 


DIFFERENT   MARINE   FORCES  333 

to  question  the  value  of  classifying  into  definite  stages  of  devel- 
opment those  deltas  which  happen  to  be  located  in  bays;  espe- 
cially when  such  classification  is  based,  not  upon  real  differences 
in  the  characteristics  of  bay  deltas  at  different  periods  of  their 
formation,  but  upon  the  non-significant  ratio  of  delta  size  to  size 
of  bay.  Gulliver  makes  no  attempt  to  divide  spits  into  stages 
of  development^'-. 

Enough  has  been  said  to  show  the  difficulty  of  classifying  the 
details  of  shore  forms  into  progressive  stages  of  systematic 
development.  It  is  clear  that,  at  least  in  the  present  state  of 
our  knowledge,  such  classification  is  neither  profitable  nor  feasible. 
This  conclusion  is  perfectly  compatible  with  the  belief  that 
shore  profiles  and  shore  outlines  pass  through  perfectly  definite 
stages  of  development,  the  proper  recognition  of  which  is  essen- 
tial to  a  full  understanding  of  shore  forms.  Gulliver  rendered  a 
valuable  service  to  physiography  by  applying  the  principles  of 
landform  evolution  to  the  study  of  shorelines  on  a  scale  never 
before  attempted.  That  he  may  possibly  have  carried  the 
attempt  too  far  does  not  affect  the  fundamental  importance  of 
his  thesis. 

Relative  Importance  of  Different  Marine  Forces  in  the  Forma- 
tion of  Bars,  Forelands,  Etc.  —  Throughout  the  discussions  of 
beaches,  spits,  bars,  tombolos,  and  forelands  which  have  occu- 
pied our  attention  on  preceding  pages,  no  special  consideration 
was  given  to  the  marine  forces  which  produced  those  forms. 
It  was  stated  that  waves  or  currents  operated  in  certain  ways, 
but  ordinarily  neither  the  methods  of  wave  action  nor  the 
kirxds  of  currents  were  discussed.  In  previous  chapters  we  have 
analyzed  the  behavior  of  waves  and  currents  of  different  types 
at  some  length;  but  it  remains  to  answer  the  important  ques- 
tion as  to  which  of  these  agencies  are  primarily  responsible  for 
the  detailed  forms  found  on  a  young  shoreline  of  submergence. 

Gulliver^^  recognizes  three  marine  agents:  waves,  tides,  and 
currents.  A  careful  reading  of  his  essay  on  ''  Shoreline  To- 
pography "  shows  that  under  "  waves  "  he  does  not  clearly 
recognize  the  highly  important  wave  currents,  but  only  the 
destructive  effects  of  wave  impact;  by  "  tides  "  he  means  tidal 
currents;  and  under  "  currents  "  he  refers  to  planetary  currents 
and  local  wind  currents.  He  is  "  inclined  to  attribute  the  attack 
of  the  sea  largely  to  the  waves,  and  its  transporting  action  largely 


334  DEVELOPMENT  OF   THE   SHORELINE 

to  the  tides  and  currents;  "  and  throughout  the  discussion  of 
individual  shore  forms  he  adheres  to  this  idea  of  transporta- 
tion largely  by  tidal,  planetary,  and  wind  currents.  There  are, 
it  is  true,  isolated  statements  which  taken  alone  seem  to  indi- 
cate a  fuller  recognition  of  wave-current  action;  but  the  treat- 
ment as  a  whole  practically  excludes  this  important  process. 
Thus  in  discussing  the  origin  of  cuspate  forelands  in  estuaries 
Gulliver  shows  that  planetary  currents  cannot  operate  in  such 
localities,  and  that  wind  currents  are  so  weak  as  to  be  over- 
powered by  the  tides;  he  therefore  concludes  that  tidal  cur- 
rents must  be  responsible  for  the  forms  in  question.  Wave 
currents  and  the  associated  "  beach  drifting"  are  not  even  re- 
ferred to  in  this  connection.  The  failure  to  recognize  the 
very  great  efficiency  of  wave  currents  in  moving  shore  debris  is 
responsible  for  the  idea,  repeatedly  expressed  in  Gulliver's 
essay^^,  that  important  longshore  transportation  does  not  take 
place  until  more  waste  is  supplied  to  the  sea  than  can  be  de- 
posited offshore.  This  might  be  true  if,  as  Gulliver  supposed, 
shore  debris  were  dependent  upon  tidal,  planetary,  and  wind 
currents  for  its  transport;  for  not  until  the  irregularities  of 
sea-bottom  and  shore  outline  have  been  measurably  smoothed 
out  by  local  deposition,  or  the  shore  has  reached  its  "  adoles- 
cent stage  "  according  to  Gulliver,  can  these  larger  currents 
sweep  uninterruptedly  along  the  coast.  Wave  currents,  how- 
ever, will  operate  effectively  on  any  shore  which  is  fronted  by 
a  body  of  water  sufficiently  large  for  the  generation  of  waves; 
and  the  most  irregular  shore  will,  even  in  its  youthful  stage, 
experience  a  very  considerable  amount  of  longshore  beach 
drifting. 

There  can  be  no  doubt  that  wave  currents  and  the  associated 
longshore  beach  drifting  play  a  very  important  role  in  the 
formation  of  various  types  of  beaches,  spits,  bars,  tombolos, 
and  forelands.  Tarr^^  has  shown  that  cuspate  forelands,  bay 
bars,  tombolos,  and  spits  are  built  by  wave  action  in  lakes  and 
nearly  tideless  bays  where  tidal  and  other  currents  are  either 
wholly  inoperative  or  far  too  weak  to  move  the  material  with 
which  the  forms  have  been  constructed.  Woodman^^  has  pre- 
sented convincing  evidence  that  in  the  Bras  d'Or  La.kes  of  Cape 
Breton  Island  cuspate  forelands,  tombolos,  bay  bars,  loop  bars, 
and  spits  are  formed  by  wave  action  without  material  aid  from 


DIFFERENT   MARINE  FORCES  335 

tidal,  wind,  or  other  currents.  Wilson's  studies",  on  the  shore 
forms  of  Lakes  Erie  and  Ontario,  and  the  Bay  of  Quinte  lead  to 
a  similar  conclusion.  The  tideless  shores  of  the  island  of  Riigen 
in  the  Baltic,  as  described  by  Philippson^^  exhibit  numerous 
spits  and  bay  bars  composed  of  material  transported  almost  ex- 
clusively by  wave  currents.  A  small  cuspate  foreland  on  the 
shore  of  Lake  George  is  described  by  Comstock^^  as  having 
been  formed  through  the  action  of  waves  generated  by  passing 
steamboats. 

Even  where  tidal  and  other  currents  not  related  to  wave  action 
move  with  high  velocity  in  the  offshore  zone,  the  waters  near 
the  shore,  where  the  forms  in  question  are  built,  commonly 
have  a  movement  too  feeble  to  transport  the  gravel  and  cobble- 
stones of  which  many  forelands  and  embankments  are  composed. 
On  the  other  hand,  wave  currents  near  the  shore  are  exceedingly 
powerful,  and  may  easily  be  observed  to  drive  the  coarsest 
debris  along  the  coast  with  a  rapidity  which  is  sometimes  sur- 
prising. Wheeler^"  repeatedly  observed  half  bricks  on  a  shingle 
beach  carried  25  to  30  yards  in  from  1|  to  2  hours,  and  quotes 
de  Ranee  as  authority  for  the  drifting  of  encaustic  tiles  by  a 
gale  for  a  distance  of  ''1  mile  in  two  tides."  Shaler  reports 
the  movement  of  pieces  of  brick  by  oblique  wave  action  at  the 
rate  of  more  than  half  a  mile  per  day.  Wind  currents  in  the 
shallow  waters  near  the  shore,  and  hydraulic  currents  generated 
by  the  combined  action  of  waves  and  wind,  while  generally  too 
feeble  to  move  coarse  debris  without  the  aid  of  wave  currents, 
frequently  co-operate  in  a  most  effective  manner  with  wave 
currents  in  causing  a  comparatively  rapid  and  exceedingly  im- 
portant longshore  transportation  of  both  fine  and  coarse  mate- 
rial. It  is  often  feasible  to  demonstrate  that  the  material  of  a 
given  foreland  or  embankment  is  derived  from  a  neighboring 
cliff,  that  beach  drifting  from  the  cliff  toward  the  area  of  accumula- 
tion proceeds  actively  under  the  influence  of  waves,  and  that  no 
other  type  of  currents  are  known  to  exist  which  are  of  sufficient 
velocity  to  move  the  debris  undergoing  transport.  It  would 
seem  logical  to  conclude  that  wave  currents  are  mainly  respon- 
sible for  the  production  of  the  forms  in  question. 

It  has  sometimes  been  held  that  the  waves  merely  agitate  the 
debris  near  the  shore  and  by  repeatedly  raising  it  from  the 
bottom  make  it  possible  for  even  weak  tidal  currents  to  effect  a 


336  DEVELOPMENT  OF  THE   SHORELINE 

longshore  transportation  of  coarse  material.     There  can  be  no 
doubt  that  tidal  and  other  currents  often  co-operate  with  wave 
currents  to  effect  the  distribution  of  shore  debris;   but  it  should 
be  remembered  that  wave  currents  are  independently  capable 
of  moving  the  coarsest  material'  along  the  shore  for  indefinite 
distances.     Gravel  and  cobblestones  would  be  carried  along  a 
coast  by  wave  currents  and  built  into  various  types  of  forelands 
and  embankments,  even  were  there  no  assistance  from  tidal  and 
other  currents;    but  these  latter  currents  would  in  general  be 
powerless  to  move  such  coarse  debris  in  the  immediate  vicinity 
of  the  shore  unless  aided  by  waves;    a   fact   fully  appreciated 
by  Gilbert*^     I  find  it  impossible,  however,  to  accept  Gilbert's 
further  conclusions  that  "  the  transporting  effect  of  waves  alone 
is  so  sHght  that  only  a  gentle  current  in  the  opposite  direction 
is  necessary  to  counteract  it,"  and  "  the  concurrence  of  waves 
and  currents  is  so  general  a  phenomenon,  and  the  ability  of 
waves  alone  is  so  small,  that  the  latter  may  be  disregarded^-." 
One  must,  however,  fully  recognize  the  possibility  that  tidal 
and  other  currents  may  be  primarily  responsible  for  the  location 
and   development   of  some  forelands   and   embankments.     For 
the  production  of  these  forms  it  is  only  necessary  that  shore 
debris  shall  be  transported  to  a  certain  locality  and  there  de- 
posited.    It  is  immaterial  what  type  of  current  accomplishes 
the  transportation.     If  tidal  currents,  or  eddy  currents,  or  cur- 
rents of  any  other  type  have  the  proper  direction  and  strength 
to  accomplish  the  observed  results,    their  possible  importance 
must  not  be  overlooked  simply  because  wave  currents  are  known 
to  have  produced  similar  results  ^Isewhere.     The  possibility  that 
certain  sandy  cuspate  forelands,  spits,  etc.,  are  primarily  the 
product  of  currents  unrelated  to  wave  action  should  especially 
be  kept  in  mind.     It  may  be  difficult,  or  even  impossible,  to 
determine  the  relative  importance  of  wave  currents  and  other 
currents  in  such  cases;  but  if  the  determination  is  at  all  possible, 
it  can  safely  be  made  only  by  one  who  studies  the   individual 
examples  in  the  field  with  an  open  mind,  and  who  is  fully  con- 
vinced of  the  ability  of  wave  currents,  as  well  as  other  more 
generally  recognized  currents,  to  produce   such   forms.     Abbe'*^ 
reports  a  case  in  which  an  eddy  current  generated  bj^  the  ebbing 
tide  seemed  to  him  to  be  responsible  for  the  development  of  a 
cusp  on  the  sandy  shores  of  Sassafras  River  in  Maryland.     As 


DIFFERENT   MARINE  FORCES 


337 


regards  my  own  studies,  I  can  say  that  I  have  found  many 
forelands  and  embankments  which  seemed  to  me  demonstrably 
due,  principally  if  not  wholly,  to  wave  currents;  but  none  which 
seemed  undoubtedly  the  product  of  other  types  of  currents.  I 
am  therefore  inclined  to  believe  that  wave  currents  have  played 
the  most  important  part  in  the  construction  of  all  the  sandy 
forelands  and  embankments  of  our  coasts. 

Davis**  refers  briefly  to  an  interesting   cuspate   bar   on   the 
south   shore   of    Lake   Balaton   in  Austria-Hungary,    the   posi- 


FiG.  86.  —  Lake  Balaton  (Flatten  Lake)  showing  position  of  cuspate  bar. 

tion  of  which  he  regards  as  evidence  that  currents  rather  than 
waves  control  the  development  of  such  forms.  On  the  basis 
of  map  study  he  concludes  that  a  promontory  which  projects  far 
into  the  lake  from  the  north  shore  probably  occasioned  the 
development  of  two  circhng  currents,  and  that  in  the  quiet 
water  between  the  two  whirls,  near  the  south  shore,  the  cuspate 
bar  was  built.  An  outline  map  of  the  lake,  showing  the  posi- 
tion of  the  bar,  is  represented  in  Figure  86.  When  due  account 
is  taken  of  the  relation  of  fetch  of  open  water  to  wave  develop- 
ment, it  will  be  seen  that  westerly  winds  must  drive  large  waves 
eastward  along  the  south  shore  of  the  western  arm  of  the  lake 


338  DEVELOPMENT  OF  THE  SHORELINE 

as  far  as  a  point  opposite  the  projecting  promontory;  but  that 
beyond  this  point  wave  action  from  the  west  will  be  weakened 
because  of  the  sheltering  effect  of  the  promontory  and  the  re- 
sulting short  stretch  of  open  water  across  which  west  winds  can 
blow.  Northerly  and  northeasterly  winds  will  drive  fairly  large 
waves  against  the  south  shore  of  the  eastern  arm  of  the  lake,  but 
just  west  of  a  point  south  of  the  promontory  there  will  be  a 
rapid  decrease  in  the  comparative  effectiveness  of  waves  from 
this  direction.  As  a  result  of  these  conditions  we  should  expect 
beach  drifting  along  the  south  shore  of  the  lake  to  be  toward  a 
point  opposite  the  promontory  for  a  considerable  distance  on 
either  side  of  that  point.  It  is  not  necessary,  therefore,  to 
assume  the  existence  in  Balaton  Lake  of  two  rotary  currents 
of  sufficient  velocity  near  the  shore  to  transport  the  debris 
which  composes  the  cuspate  bar,  for  a  bar  opposite  the  promon- 
tory is  a  perfectly  normal  and  expectable  result  of  wave  action 
alone.  It  may  even  be  shown  that  wind  waves  from  a  single 
direction  are  alone  competent  to  build  a  cuspate  bar  at  the 
point  in  question. 

The  frequent  appeal  to  a  pair  of  hypothetical  circling  cur- 
rents or  eddies  with  a  triangular  space  of  comparatively  dead 
water  between  the  shore  and  the  point  of  tangency  of  the  eddies, 
in  order  to  explain  the  development  of  cuspate  bars  and  fore- 
lands, has  long  seemed  to  the  writer  unnecessary,  and  insuft- 
ciently  justified  by  evidence  of  critical  value.  In  many  cases 
there  is  ample  evidence  that  currents  of  some  type  effect  the 
longshore  movement  of  debris  either  toward  or  away  from  the 
point  of  the  cusp;  but  in  most  cases  the  only  basis  for  the  sup- 
posed pair  of  circling  currents  is  the  assumption  that  they  are 
required  in  order  to  explain  the  presence  of  a  foreland  of  cuspate 
form. 

Both  the  development  of  cuspate  bars  and  forelands,  and  the 
longshore  movement  of  debris  causing  offsets,  overlaps,  and  stream 
deflections,  may  usually  be  explained  as  the  normal  product  of 
longshore  beach  drifting,  assisted  by  the  wind  currents  and 
hydraulic  currents  which  ordinarily  accompany  that  process. 
Thus  the  Darss  foreland  (Fig.  131)  has  been  built  with  debris 
drifted  eastward  by  the  action  of  waves  generated  under  west- 
erly winds,  the  beach  drifting  no  doubt  having  been  supple- 
mented by  water  forced  eastward  by  the  friction  of  the  wind 


MATURE   STAGE  339 

on  the  sea  surface,  and  by  eastward  moving  h3'draulic  currents 
which  would  attempt  to  remove  the  water  piled  against  the 
coast  by  wind  and  waves.  As  the  point  of  the  Darss  advanced 
northward  it  gradually  sheltered  the  waters  to  the  eastward  from 
westerly  winds,  and  gave  the  waves  generated  by  easterly  winds, 
formerly  overpowered  by  the  dominant  action  from  the  west,  an 
opportunity  to  determine  the  movement  of  shore  debris.  Con- 
sequently beach  drifting  from  east  to  west  has  apparently  pre- 
vailed east  of  the  northward  projecting  point  of  the  Darss  in 
recent  times,  and  has  doubtless  caused  the  westward  deflection 
of  the  Prerow  River.  In  a  similar  manner  the  Carolina  capes, 
regarded  by  Gulliver^^  and  Davis^'^  as  having  been  built  between 
pairs  of  circling  currents,  may  be  explained  as  the  expectable 
result  of  normal  wave  action.  In  neither  case,  nor  in  any  other 
known  to  the  writer,  does  it  seem  necessary  to  assume  the  exist- 
ence of  pairs  of  rotary  currents,  the  evidence  for  which  is  either 
inconclusive  or  wholly  lacking. 

Mature  Stage.  —  We  have  inquired  at  some  length  into  the 
series  of  forms  which  characterize  the  young  shoreline  of  sub- 
mergence, and  have  found  that  the  unorganized  condition  of 
current  action  along  such  a  shore  combines  with  the  initial 
irregularities  of  the  submerged  land  area  to  produce  an  almost 
endless  variety  of  interesting  shore  features.  In  striking  con- 
trast with  the  complexity  and  variety  of  youth  is  the  simplicity 
of  the  mature  shoreline  of  submergence  (Fig.  88).  Let  us 
trfice  briefly  the  steps  by  which  that  simplicity  is  attained. 
During  its  initial  stage  a  shoreline  of  submergence  is  wholly 
unadjusted  to  the  waves  and  currents  which  operate  upon  it. 
Waves  break  irregularly  upon  the  uneven  bottom  and  against 
the  complicated  shoreline;  currents  are  split  up  and  deflected 
in  every  conceivable  direction,  and  any  branch  current  may 
find  itself  flowing  swiftly  against  some  headland  or  over  some 
shallow  at  one  moment,  and  dropping  its  load  a  moment  later 
when  its  velocity  is  checked  upon  passing  into  deep  water 
opposite  some  bay.  This  unadjusted  condition  continues  in 
constantly  diminishing  degree  throughout  the  youth  of  the 
shoreline  of  submergence  and  is  characteristic  of  that  stage  of 
its  development,  just  as  an  irregular  longitudinal  profile  is  char- 
acteristic of  the  young  stage  of  stream  development.  But  the 
removal   of   outlying   islands,    the    cutting   back   of   projecting 


340  DEVELOPMENT  OF  THE   SHORELINE 

headlands,  and  the  building  of  bars  across  the  mouths  of  bays, 
gradually  simplify  the  outer  shoreline  and  permit  longshore 
currents  to  move  through  greater  distances  unimpeded  by  pro- 
jecting land  masses.  At  the  same  time  wave  action  has  estab- 
lished the  shore  profile  of  equilibrium  on  the  seaward  side  of  the 
bars  and  is  working  toward  the  same  end  on  the  cliffed  head- 
lands. Thus  the  shoreline  progresses  toward  maturity.  When 
the  headlands  are  partially  cut  back  and  many  of  the  interven- 
ing bays  are  nearly  or  quite  closed  by  bars,  so  that  longshore 
currents  may  move  through  considerable  distances  before  en- 


FiG.  87.  —  Shoreline  of  submergence,  submature  stage. 

countering   obstructions,    the   shoreline   may   be   said   to   have 
reached  late  youth  or  submaturity  (Fig.  87). 

Still  later  the  headlands  will  be  so  far  cut  back  and  bay-mouth 
bars  will  be  so  uniformly  present  opposite  the  original  re-entrant 
angles  of  the  coast,  that  a  very  simply  curved  or  nearly  straight 
shoreline  will  permit  longshore  currents  to  transport  debris  for 
indefinite  distances  without  hindrance.  The  shoreline  is  now 
nicel}^  adjusted  to  the  forces  operating  upon  it;  the  beach 
profile  of  equilibrium  is  fully  established  both  on  the  seaward 
side  of  the  bars  and  at  the  bases  of  the  retreating  cliffs;  and 
while  the  cliff  profile  may  still  be  too  steep  to  permit  one  to  call 
the  entire  shore  profile  mature,  the  shore  outline  is  such  that 
debris  moves  with  the  longshore  currents  as  systematically  as 


MATURE  STAGE 


341 


sediment  is  moved  seaward  on  the  nicely  adjusted  slope  of  a 
graded  river.  In  short,  the  shoreline  itself  has  reached  a  graded 
condition;  and  this  condition  has  been  attained  by  an  orderly 
process  of  cutting  back  the  headlands  and  bridging  the  bays 
with  the  resulting  debris,  just  as  the  grading  of  the  river  is 
accomplished  by  cutting  down  projecting  rock  masses  and  fill- 
ing depressions  with  the  erosion  products.  The  establishment 
of  the  graded  condition  marks  the  entrance  of  either  river  or 
shoreline  into  the  early  mature  stage  of  its  development. 

Full  maturity  (Fig.  88)  is  attained  only  when  the  shoreline  has 
been  pushed  inland  beyond  the  bay  heads,  and  lies  against  the 


Fig.  88.  —  Shoreline  of  submergence,  mature  .stage. 


original  mainland  throughout  all  its  course.  By  this  time  the 
numerous  islands  and  prominent  headlands  of  youth  have  been 
obliterated,  the  great  variety  of  spits,  bars,  tombolos,  and  fore- 
lands have  disappeared,  bay  deltas  and  marsh  deposits  have 
been  consumed  by  the  advancing  waves,  and  there  remains 
only  a  comparatively  narrow  beach  at  the  base  of  an  almost 
contiinious  marine  cliff  which  borders  a  shoreline  of  very  simple 
curvature.  Monotony  rather  than  variety  is  the  distinguishing 
feature  of  maturity,  although  the  cliffs,  often  covered  with  vege- 
tation, may  be  of  such  magnitude  as  to  impart  majestic  grandeur 
to  the  coastal  scenery. 

Even  in  the  maturity  of  a  shoreline  of  submergence  the  ad- 


342 


DEVELOPMENT  OF  THE  SHORELINE 


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MATURE  STAGE  343 

vance  of  the  waves  may  be  so  rapid  that  stream  erosion  cannot 
lower  valley  floors  as  fast  as  the  shoreline  is  cut  back.  This  is 
especially  apt  to  be  the  case  where  streams  are  small  and  weak, 
and  where  the  nature  of  the  country  rock  permits  much  under- 
ground seepage  and  little  surface  erosion.  Hanging  valleys  due 
to  this  cause  are  developed  on  a  small  scale  along  many  coasts 
(Plate  XLIII),  but  are  especially  striking  on  the  mature  coast 
of  northwestern  France  (Plate  XXI),  where  many  valleys  are 
left  hanging  in  the  face  of  the  chalk  cliffs  because  wave  erosion 


Fig.  89.  —  Valleases  on  the  northwest  coast  of  France. 

cuts  the  cliffs  backward  faster  than  the  smaller  streams  can  cut 
their  valleys  downward.  These  hanging  valleys  have  been  given 
a  special  name,  "  valleuses,"  by  the  French,  and  by  their  frequent 
convergence  toward  a  point  some  distance  out  to  sea  (Fig.  89), 
they  furnish  an  indication  of  the  extensive  wave  erosion  which 
has  removed  the  main  valley  they  once  united  to  form.  Only 
the  larger  streams  have  been  able  to  reduce  their  gently  sloping 
valley  floors  to  sealevel  as  fast  as  the  waves  cut  inland,  and  their 
valleys  form  the  only  interruptions  in  a  line  of  cliff  which  extends 
for  many  miles  in  very  simple  curves.  Where  streams  are  un- 
able to  reduce  a  whole  broad  valley  as  rapidly  as  the  shoreline  is 


344  DEVELOPMENT  OF  THE  SHORELINE 

worn  back,  we  may  find  a  narrow  gorge  cut  in  the  bottom  of 
the  broad  valley,  the  bottom  of  the  gorge  alone  being  reduced  to 
an  accordant  junction  with  the  sea. 

Not  all  parts  of  a  shoreline  develop  at  the  same  rate.  Along 
weak  rock  coasts  maturity  is  attained  more  quickly  than  along 
coasts  composed  of  more  resistant  material.  Even  after  matur- 
ity is  attained  the  shoreline  on  a  broad  belt  of  weak  rocks  will 
retreat  more  rapidly  than  adjacent  sections,  until  the  depth  of 
the  indentation  has  so  weakened  wave  attack  that  a  condition 
of  equilibrium  is  attained.  Thereafter  all  parts  of  the  shore- 
line retreat  at  the  same  rate,  the  portion  bordering  the  weak 
rock  area  keeping  a  constant  distance  in  advance  (farther  in- 
land). The  initial  more  rapid  retrogression  of  the  shoreline 
on  weak  rocks  depends  primarily  on  two  factors:  in  the  first 
place  the  weak  rocks  jdeld  more  readily  to  wave  attack;  and  in 
the  second  place,  weak  rock  areas  are  normally  worn  down  nearer 
to  sealevel  by  subaerial  agencies,  with  the  result  that  waves 
and  currents  have  to  dispose  of  much  less  debris  than  they  do 
where  high  cliffs  shed  vast  quantities  of  waste  upon  a  slowly 
retrograding  beach.  A  mature  coast  should  therefore  show 
simple  but  distinct  curves  systematically  related  to  rock  struc- 
ture. 

Exposed  shorelines  develop  more  rapidly  than  do  protected 
shorelines,  a  fact  well  illustrated  by  the  more  advanced  stage  of 
development  reached  on  the  Atlantic  coast  of  the  Maryland- 
Delaware  coastal  plain,  as  compared  with  the  Chesapeake  Bay 
coast  of  the  same  district.  Other  factors  likewise  retard  or 
accelerate  shoreline  development,  with  the  net  result  that  a 
shoreline  approaching  maturity  may  consist  of  a  series  of  more 
or  less  isolated  stretches  in  a  mature  stage,  separated  by  other 
stretches  which  are  still  submature  or  even  young.  As  the 
waves  cut  farther  into  the  land  the  mature  sections  increase  in 
length,  and  finally  unite  when  the  entire  shoreline  has  attained 
full  maturity. 

Old  Stage.  —  The  old  age  of  a  shoreline  of  submergence  does 
not  differ  essentially  from  the  old  age  of  a  shoreline  of  emergence. 
It  will  be  more  convenient,  therefore,  to  postpone  discussion  of 
this  stage  of  development  until  after  the  youth  and  maturity 
of  shorelines  of  emergence  have  been  considered. 


REFERENCES  345 


RESUME 

In  the  present  chapter  we  have  passed  in  review  those  shore 
forms  which  characterize  the  different  developmental  stages  of 
shorehnes  of  submergence.  It  has  been  shown  that  by  far  the 
greatest  variety  of  forms  is  associated  with  young  shorehnes  of 
this  class;  while  in  the  mature  and  old  stages  the  forms  are  fewer 
in  number,  more  simple  in  character,  and  more  nearly  like  those 
in  the  corresponding  stages  of  other  classes  of  shorelines.  Special 
consideration  has  been  given  to  the  question  as  to  how  far  it  is 
wise  to  attempt  the  classification  of  minor  details  of  shore  form 
into  successive  stages  of  development,  and  reasons  presented  in 
support  of  the  opinion  that  these  minor  forms  should  not  be  so 
classified.  The  origin  of  the  various  shore  forms,  including  cus- 
pate  bars  and  forelands,  h\ye  been  examined  with  some  care  in 
order  to  discover  which  mahine  forces  are  primarily  responsible 
for  their  development;  and \  the  conclusion  has  been  reached 
that  beach  drifting  under  the;  influence  of  wind-formed  waves  is 
more  potent  in  their  constmciiin  than  are  tidal  and  other  cun-ents. 
We  are  now  prepared -fo  turn  our  attention  to  shorelines  of  emer- 
gence, and  to  discuss  the  special  forms  characteristic  of  their 
successive  developmental  stages. 


REFERENCES 

1.  Beche,  Henry  T.  de  la.      Researches  in  Theoretical  Geology,  p.  193, 

London,  1834. 

2.  Dana,  J.  D.     Geology,  U.  S.  Exploring  Expedition  during  the  Years 

1838  to  1842  under  the  Command  of  Charles  Wilkes.     X,  393,  1849. 

3.  Davis,  W.  M.     Die  Erkhlrende  Beschreibung  der  Landformen,  p.  493, 

Leipzig  and  Berlin,  1912. 

4.  Da\is,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  704,  Boston,  1909. 

5.  Gilbert,   G.   K.     The  Topographic   Features  of   Lake  Shores.     U.   S, 

Geol.  Surv.,  5th  Ann.  Rept.     P.  92,  1885. 

6.  GuLLi\^R,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  242,  1899. 

7.  Hind,  H.  Y.      Report  on  the  Preservation  and  Improvement  of  Toronto 

Harbor.     Canadian  Journal.     II,  Supplement,  pp.  1-14,  1854. 

8.  Fleming,  Sanford.     Toronto  Harbor— Its  Formation  and  Preserva- 

tion.    Canadian  Journal,  II,  105-107,  223-230,  18.53. 

9.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  715,  Boston,  1909. 


346  DEVELOPMENT  OF  THE  SHORELINE 

10.  Johnson,   Douglas  W.  and  Reed,   W.  G.     The  Form  of  Nantasket 

Beach.     Jour,  of  Geol.     XVIII,  162-189,  1910. 

11.  DuANE,   J.   C,   ti  al.     Repor     of   Board  of  Engineers   (on  Deepening 

Gedney's  Channel  Through  Sandy  Hook  Bar,  New  York).  48th 
Congress,  2nd  Session,  House  of  Representatives.  Executive  Docu- 
ment No.  78,  p.  12,  1885. 

12.  Gulliver,  F.  P.     ShoreUne  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  213,  1899. 

13.  RiCHTHOFEN,  F.  VON.     Flihrer  fiir  Forschungsreisende,  p.  181,  Hannover, 

1901. 

14.  Hentzschel,   Otto.     Die  Hauptkiistentypen  des  Mittelmeers,  p.   52, 

Leipzig,  1903. 

15.  Shaler,   N.   S.     Beaches  and  Tidal   Marshes   of  the  Atlantic   Coast. 

National  Geogr.  Monogr.     I,  p.  153,  1895. 

16.  Gulliver,  F.  P.     Shorehne  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  179,  1899. 

17.  Branner,  J.  C.     Stone  Reefs  on  the  Northeast  Coast  of  Brazil.     Bull. 

Geol.  Soc.  Amer.     XVI,  1-12,  1905. 
Branner,  J.  C.     The  Stone  Reefs  of  Brazil:   Their  Geological  and  Geo- 
graphical Relations,  with  a  Chapter  on  the  Coral  Reefs.     Bull.  Mus 
Comp.  Zool.     XLIV,  1-285,  1904. 

18.  Beaufort,  Francis.     Karamania,  or  a  Brief  Description  of  the  South 

Coast  of  Asia  Minor  and  of  the  Remains  of  Antiquity,  pp.  174-182, 
London,  1817. 

19.  Cold,  Conrad.     Kiisten-Veranderungen  im  Archipel,  p.  31,  Marburg, 

1886. 

20.  Gilbert  G.  K.     Lake  Bonneville.      U.  S.  Geol.  Surv.  Mon.     I,  p.  55, 

1890. 

21.  Gulliver,  F.  P.     ShoreUne  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  189,  1899. 

22.  Pianigiani,    O.     Dizionario   Etimologico   della    Lingua   Italiana.     II, 

1438,  1907. 

23.  Gulliver,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  195,  1899. 

24.  Johnson,  D.  W.  and  Reed,  W.  G.     The  Form  of  Nantasket  Beach. 

Jour,  of  Geol.     XVIII,  162-189,  1910. 

25.  HoBBs,  W.  H.     Earth  Features  and  Their  Meaning,  p.  241,  New  York, 

1912. 

26.  FLEivnNG,  Sanford.     Toronto  Harbor  —  Its  Formation  and  Preserva- 

tion.    Canadian  Jour.     II,  227,  1853. 

27.  GuuLn'ER,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  217,  218,  1899. 

28.  Qi7LLi\T]R,  F.  P.     Cuspate  Forelands.     Bull.  Geol  Soc.  Amer.     VII,  203, 

1896. 

29.  Gulliver,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  191,  1899. 

30.  Ibid.,  p.  192. 

31.  Ibid.,  pp.  193,  199. 


REFERENCES  347 

32.  Gulliver,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts,  and 

Sciences.     XXXIV,  204-226,  1899. 

33.  Ihid.,  pp.  174,  178,  183,  216. 

34.  lUd.,  pp.  177,  192,  216. 

35.  Tarr,   R.   S.     Wave-formed  Cuspate  Forelands.     American  Geologist. 

XXII,  1-12,  1898. 

36.  Woodman,  J.  E.     Shore  Development  in  the  Bras  d'Or  Lakes.      Ameri- 

can Geologist.     XXIV,  329-342,  1899. 

37.  Wilson,  A.  W.  G.     Shoreline  Studies  on  Lakes  Ontario  and  Erie.     Bull. 

Geol.  Soc.  Amer.     XIX,  493,  1908. 
Wilson,  A.  W.  G.     Cuspate  Forelands  Along  the  Bay  of  Quuite.     Jour, 
of  Geol.     XII,  111,  1904. 

38.  Philippson,  Alfred.     Uber  die  Kiistenformen  der  Insel  Rligen.     Ver- 

handl.  d.  Naturhist.     Vereins  XLIX,  63-72,  1892. 

39.  CoMSTOCK,  F.  N.     An  Example  of  Wave  Formed  Cusp  at  Lake  George, 

N.  Y.     Amer.  Geologist.     XXV,  192,  1900. 

40.  Wheeler,  W.  H.     The  Sea  Coast:    Destruction:    Littoral  Drift:    Pro- 

tection, p.  38,  London,  1902. 

41.  Gilbert,  G.  K.     The  Topographic  Features  of  Lake  Shores.     U.  S.  Geol. 

Surv.  5th  Ann.  Rep.,  p.  85,  1885. 

42.  Ihid.,  p.  86. 

43.  Abbe,  Cleveland.     A  General  Report  on  the  Physiography  of  Mary- 

land.    Maryland  Weather  Service.     I,  99,  1899. 

44.  Davis,   W.    M.       Die  Erklarende   Beschreibung   der  Landformen,    pp. 

505-506,  Leipzig  and  Berlin,  1912. 

45.  Gulliver,    F.   P.     Cuspate  Forelands.     Bull.   Geol.   Soc.   Amer.     VII, 

407-410,  1896. 

46.  Davis,   W.   M.       Die  Erklarende   Beschreibung  der  Landformen,   pp. 

475-477,  Leipzig  and  Berlin,  1912. 


CHAPTER  VII 
DEVELOPMENT   OF  THE   SHORELINE  (Continued) 

B.     SHORELINES   OF   EMERGENCE 

Advance  Summary.  —  The  method  of  treatment  followed  in 
the  preceding  chapter  is  here  applied  to  shorelines  of  emergence. 
Features  characteristic  of  the  youth,  maturity  and  old  age  of 
shorelines  of  this  class  are  described,  and  special  emphasis  placed 
upon  those  forms  which  for  any  reason  merit  extended  consid- 
eration. Thus  the  origin  of  the  offshore  bar  is  quite  fully  dis- 
cussed, and  new  evidence  presented  to  test  conflicting  theories. 
The  history  of  tidal  inlets  is  traced  in  some  detail,  and  in  view 
of  their  behavior  modifications  of  the  current  explanations  of 
offshore  bar  development  are  suggested.  It  is  shown  that  there 
exists  a  significant  relationship  between  the  positions  of  head- 
lands to  which  some  offshore  bars  are  attached,  the  direction  of 
longshore  currents,  the  distribution  of  inlets,  the  width  of  lagoon 
and  the  extent  of  lagoon  filling;  and  an  explanation  of  this  in- 
teresting relationship  is  offered.  The  effect  of  coastal  subsidence 
and  coastal  elevation  upon  the  history  of  the  offshore  bar  and 
lagoon  are  discussed,  and  the  fallacy  of  the  theory  that  offshore 
bars  are  an  evidence  of  coastal  subsidence  is  exposed.  Such  an 
account  is  given  of  the  changes  to  which  offshore  bars,  tidal 
inlets  and  lagoons  are  commonly  subject,  as  will,  it  is  hoped, 
prove  of  value  to  the  harbor  and  marine  engineer  as  well  as  to 
the  geographer  and  geologist. 

Initial  Stage.  —  When  a  sea-bottom  or  a  lake-bottom  emerges 
from  beneath  the  water,  either  because  of  an  uplift  of  the  land 
or  a  sinking  of  the  water  surface,  the  new  shoreline  may  be 
called  a  "shoreline  of  emergence."  The  essential  characteris- 
tics of  such  a  shoreline  depend  upon  the  fact  that  the  bottoms 
of  lakes  and  seas  are  not  subjected  to  the  river  erosion  which 
roughens  land  surfaces,  but  on  the  contrary  are  made  even 
smoother  by  the  continual  deposition  of  matter  brought  into 
these  quiet  water  bodies.     If  the  plain  of  deposition  emerges, 

348 


INITIAL  STAGE  Z^Q-  ■ 

the  water  surface  coming  to  rest  against  any  portion  of  the 
nearly  level  plain  surface,  will  give  a  straight  oi"  nearly  straight 
shorehne. 

In  case  the  bottom  of  a  sea  or  lake  represents  a  rugged  land 
area  but  recently  depressed,  and  the  emergence  occurs  before 
deposition  has  had  an  opportunity  to  bury  the  inequalities  and 
produce  a  smooth  subaqueous  surface,  then  the  emergence  of 
the  rugged  bottom  will  give  an  irregular  shoreline.  Evidently 
the  dominant  features  of  this  shoreline  were  determined  by  the 
submergence  of  the  original  hills  and  valleys,  and  not  by  the  later 
partial  emergence  which  was  insufficient  to  change  the  type  of 
the  shoreline  already  existing.  For  pu''poses  of  classification  and 
study  such  a  shoreline  must  be  grouped  with  shorelines  of  sub- 
mergence, the  partial  emergence  being  of  secondary  importance 
only.  Thus  the  coast  of  Maine  is  an  excellent  example  of  a 
shoreline  of  submergence,  although  a  moderate  amount  of  emer- 
gence succeeded  the  submergence  which  gave  the  shore  its  essen- 
tial characters. 

A  subaqueous  plain  of  deposition  normally  has  a  surface 
gently  inclined  away  from  the  shoreline.  After  emergence, 
therefore,  we  should  expect  to  find  shallow  water  seaward  from 
the  new  shoreline  of  emergence,  the  offshore  slope  being  very 
gradual  This  is  one  of  the  essential  characteristics  of  the  initial  < 
stage  of  a  shoreline  of  emergence;  and  since  the  shallowness 
of  the  water  prevents  the  access  of  large  waves  to  the  shore,  the 
early  stages  of  development  of  such  a  shoreline  are  much  affected 
by  this  feature. 

During  storms  large  waves  break  far  out  to  sea,  sometimes 
encountering  water  too  shallow  for  their  propagation  several 
miles  from  the  shoreline.  Smaller  waves  reach  the  shore  and 
begin,  their  attack  upon  the  land.  A  cliff  is  cut,  which,  because 
of  its  small  size,  is  sometimes  called  a  nip  in  the  edge  of  the  land. 
•In  the  manner  fully  explained  on  previous  pages  the  bench  in 
front  of  this  small  cliff  is  gradually  deepened  and  the  cliff  pushed 
inland,  increasing  in  height  as  it  is  cut  farther  into  the  upward 
sloping  coastal  plain.  In  the  meantime  the  large  waves  break- 
ing farther  seaward  are  cutting  into  the  sea-bottom,  and  while 
part  of  the  resulting  debris  is  carried  out  to  deeper  water,  another 
part  is  thrown  upon  the  landward  edge  of  the  submarine  cut, 
to  form  a  submarine  bar  roughly  parallel  with  the  shoreline. 


350  DEVELOPMENT  OF   THE   SHORELINE 

Where  emergence  is  gradual,  as  is  perhaps  usually  the  case,  the 
bar  may  form  before  the  mainland  is  appreciably  cliffed  by 
wave  action,  and  the  nip  observed  later  may  then  be  wholly 
the  work  of  lagoon  waves.  When  the  bar  has  been  built  upward 
to  the  water  surface,  the  shoreline  of  emergence  may  be  said  to 
have  passed  its  initial  stage  and  to  have  entered  that  of  youth. 

Young  Stage.  —  As  soon  as  the  submarine  bar  lying  offshore 
has  been  raised  above  the  surface  of  the  water,  we  can  distin- 
guish an  outer  and  an  inner  shoreline;  the  first  bordering  the 
seaward  side  of  the  offshore  bar,  or  barrier  beach  as  it  is  often 
called,  while  the  second  is  the  original  shoreline,  now  character- 
ized by  the  low  cliff  or  nip  bordering  the  edge  of  the  mainland. 
Between  the  mainland  and  the  bar  lies  a  lagoon,  on  whose  sur- 
face small  waves  only  can  be  generated,  both  because  of  the 
shallow  depth  and  the  comparatively  short  stretch  of  open 
water  exposed  to  wind  action.  Inasmuch  as  the  offshore  bar 
is  the  most  striking  feature  of  the  young  shoreline  of  emergence, 
we  may  appropriately  consider  the  precise  method  of  its  devel- 
opment somewhat  fully. 

Offshore  Bar.  —  Various  theories  have  been  offered  to  ac- 
count for  the  production  of  a  narrow  bar  lying  parallel  to,  but 
some  distance  from,  a  gently  sloping  sandy  shore.  One  writer 
has  even  gone  so  far  as  to  deny  their  marine  origin.  Bryson^, 
writing  in  1888,  considered  that  the  offshore  bars  along  the 
south  side  of  Long  Island  had  been  produced  by  subglacial 
streams,  and  naively  remarks:  "  These  beaches  ha^e  generally 
been  held  to  be  of  marine  origin,  but  this  idea  is  being  aban- 
doned." In  a  later  paper^  he  states  that  the  bars  are  really 
kames.  We  can  at  least  agree  with  his  admission  that  "  this 
hardly  seems  possible."  Schott^  tried  to  explain  the  remark- 
able offshore  bar  bordering  the  north  shore  of  Yucatan  as  the 
product  of  outward  pressing  land  waters  meeting  the  resistance 
of  the  sea. 

Louis  Agassiz"*  suggested  that  at  least  along  the  coast  of' 
the  southern  United  States  the  offshore  bars  of  sand  rested 
upon  pre-existing  coral  reefs.  MerrilP  was  convinced  that 
these  bars  were  "  formed  under  water  by  wave  and  current 
action,"  but  experienced  difficulty  in  accounting  for  the  appear- 
ance of  their  crests  above  the  surface  of  the  water.  He  solved 
the  problem  by  assuming  an  elevation  of  the  sea-bottom  which 


YOUNG  STAGE  351 

"  brought  these  sand-bars  above  water  into  a  horizon  of  seohan 
action.  Once  above  the  sea,  the  beaches  would  maintain  their 
existence."  McGee^  on  the  other  hand,  seems  to  have  regarded 
the  presence  of  offshore  bars  and  keys  as  a  proof  of  coastal  sub- 
sidence, the  sea  having  encroached  upon  the  land  so  rapidly  as 
a  consequence  of  the  sinking  movement  that  the  bars  were  left 
behind.  This  implies  the  belief  that  such  bars  begin  to  form 
at  the  edge  of  the  mainland,  which  is  clearly  the  conception  of 
Ganong^  who  writes  as  follows  concerning  small  bars  off  the  . 
coast  of  New  Brunswick:  "  They  no  doubt  formed  against  the 
margin  of  the  flat  upland  as  ordinary  shore  beaches.  But  the 
steadily  progressing  subsidence  carried  the  land  beneath  the  sea 
faster  than  the  beaches,  whose  rate  of  inward  movement  is 
largely  determined  by  the  rate  of  erosion  of  the  protecting  head- 
lands, could  follow;  hence  the  lagoons  were  formed."  While 
the  forms  described  by  Ganong  should  perhaps  be  classed  as 
bay  bars,  the  principle  involved  does  not  differ  from  that  in  the 
case  of  the  offshore  bars  called  "  keys  "  by  McGee.  It  would 
seem  that  a  similar  idea  as  to  the  origin  of  offshore  bars  has  been 
entertained  by  David  White  and  C.  A.  Davis,  as  it  is  otherwise 
difficult  to  understand  their  belief  that  such  bars  should  be  re- 
garded as  proofs  of  coastal  subsidence^. 

One  of  the  best  accounts  of  offshore  bars  is  of  earlier  date 
than  any  of  the  discussions  mentioned  above,  having  been 
published  by  Elie  de  Beaumont^  in  1845.  In  his  "  Legons  de 
Geologie  Pratique  "  this  keen  observer  not  only  describes  the 
bars  at  much  length  and  explains  how  wave  action  on  a  shallow 
bottom  removes  part  of  the  material  and  heaps  it  up  in  a  ridge 
parallel  to  the  shore,  but  also  states  that  this  change  involves  a 
readjustment  of  the  submarine  slope  to  bring  it  into  closer 
harmony  with  the  movements  of  the  water.  In  other  words, 
he  recognizes  the  effort  of  the  sea  to  establish  a  profile  of  equi- 
librium, and  that  the  offshore  bar  is  one  result  of  this  effort. 

Shaler'"  emphasizes  the  relation  of  offshore  bars  to  shorelines 
of  emergence,  and  assumes  an  uplift  of  the  continental  shelf  as 
the  first  step  leading  to  the  formation  of  such  a  bar.  The 
second  step  is  considered  by  him  to  be  shallowing  of  the  offshore 
zone  by  deposition  of  debris  eroded  from  the  margin  of  the 
land,  and  other  debris  moved  landward  by  the  friction  of  the 
waves  upon  the  bottom  farther  seaward.     Not  until  this  shal- 


/ 


352        DEVELOPMENT  OF  THE  SHORELINE 

lowing  has  occurred  does  he  imaghie  bar  formation  to  begin. 
Storm  waves  then  break  at  a  considerable  distance  from  the 
land,  and  drop  the  debris  they  were  moving  landward,  thus 
Imilding  a  ridge  parallel  with  the  shore  which  is  permanently 
preserved  in  case  it  rises  above  the  surface  of  the  sea. 

Gilbert*^  and  RusselP-  do  not  appear  to  make  a  clear  dis- 
tinction between  bay  bars  and  offshore  bars.  Thus  Gilbert's 
description  of  what  he  terms  "  the  barrier  "  would  seem  to  apply 
to  offshore  bars  formed  in  gradually  shallowing  water.  This 
interpretation  is  sustained  by  the  fact  that  he  compares  with 
"  the  barrier,"  those  "  low  ridges  of  sand  or  gravel  running  par- 
allel to  the  shore  and  entirely  submerged  "  which  can  be  traced 
continuously  for  hundreds  of  miles  along  the  shores  of  Lake 
Michigan,  but  whose  origin  is  uncertain^\  On  the  other  hand, 
the  bay  bar  at  Stockton,  Utah,  is  called  both  a  "  bay  bar  "  and 
a  "  barrier^*;  "  and  the  dependence  upon  shore  drift  ascribed 
to  "  barriers  "  would  seem  more  characteristic  of  bay  bars  than 
of  offshore  bars.  RusselP^  describes  the  formation  of  "  barrier- 
bars  "  in  terms  which  recall  Gilbert's  description  of  "  the  barrier  " ; 
and  compares  them  with  the  submerged  ridges  paralleling  the 
Lake  Michigan  shores.  But  Russell's  illustration  of  "  barrier- 
bars  "  shows  ordinary  bay  bars  closing  the  mouth  of  a  small  bay. 

Assuming  that  Gilbert  and  Russell  intended  their  descriptioi^s 
to  apply  equally  to  offshore  bars  and  bay  bars,  and  taking 
Gilbert's  description  for  examination  as  being  the  more  complete 
of  the  two,  we  may  next  note  the  essential  elements  of  this  theory 
of  offshore  bar  formation.  According  to  Gilbert  the  material 
of  which  the  bar  is  composed  consists  of  "shore  drift  "  which 
is  being  moved  parallel  to  the  coast  by  longshore  currents.  On 
a  gradually  shallowing  shore  "  the  waves  break  at  a  consider- 
able distance  from  the  water  margin.  The  most  violent  agita- 
tion of  the  water  is  along  the  line  of  breakers;  and  the  shore 
drift,  depending  upon  agitation  for  its  transportation,  follows 
the  line  of  the  breakers  instead  of  the  water  margin.  It  is 
thus  built  into  a  continuous  outlying  ridge  at  some  distance 
from  the  water's  edge.  .  .  .  The  barrier  is  the  functional 
equivalent  of  the  beach.  .  .  .  The  beach  and  the  barrier  are 
absolutely  dependent  on  shore  drift  for  their  existence.  If  the 
essential  continuous  supply  of  moving  detritus  is  cut  off,  .  .  . 
the  structure  (is)  demolished  by  the  waves  which  formed  it^^." 


YOUNG  STAGE 


353 


354  DEVELOPMENT  OF  THE   SHORELINE 

Davis  follows  Shaler  in  relating  the  offshore  bar  to  a  shore- 
line of  emergence,  but  does  not  admit  the  necessity  of  shallowing 
by  deposition  before  bar  formation  can  commence.  He  follows 
de  Beaumont  and  Shaler  in  deriving  the  material  of  the  bar 
from  the  offshore  bottom,  and  disagrees  with  Gilbert  who  re- 
gards the  material  of  the  bar  as  shore  debris  in  process  of  trans- 
portation parallel  to  the  shore;  for  while  Gilbert  believed  long- 
shore transportation  to  be  absolutely  necessary,  Davis  states 
his  conviction  that  offshore  bars  "  might  be  developed  essen- 
tially under  the  control  of  on-  and  offshore  action  alone'^" 
The  successive  stages  in  the  development  of  an  offshore  bar 
are  described  by  Davis  at  some  length  in  his  "  Erklarende 
Beschreibung  der  Landformen,"  where  the  discontinuous  char- 
acter of  the  bar  during  its  initial  stage,  and  the  progressive 
narrowing  of  tidal  inlets  to  a  limiting  size  determined  by  an 
ultimate  equilibrium  between  tidal  and  longshore  currents,  are 
emphasized^*.  Shaler^^,  on  the  other  hand,  believed  that  the 
offshore  bar  had  great  continuity  when  first  formed  and  that 
the  so-called  tidal  inlets  were  really  "  outlets  "  formed  by  the 
bursting  through  of  land  waters  dammed  off  from  the  sea  by 
the  bar. 

Agassiz's  theory,  connecting  offshore  sand  bars  with  coral 
reefs,  may  be  dismissed  on  the  ground  that  records  of  numerous 
wells  drilled  on  the  offshore  bars  along  the  coast  of  the  south- 
eastern United  States  fail  to  show  the  presence  of  such  a  reef 
below  the  sandy  surface.  While  it  is  true  that  coral  limestone 
sometimes  underlies  a  ridge  of  beach  or  dune  sand,  as  for  ex- 
ample in  the  Florida  keys,  such  a  relation  is  not  typical  for  the 
offshore  bars  from  Long  Island  and  New  Jersey  to  Texas.  Both 
theoretical  considerations,  and  direct  observations  of  small  off- 
shore bars  raised  above  the  level  of  lakes  by  wave  action  alone, 
justify  us  in  rejecting  Merrill's  contention  that  an  elevation  of 
the  sea-bottom  is  necessary  to  bring  the  bar  crest  above  water. 
Equally  untenable  is  the  position  of  McGee,  Ganong,  White, 
and  C.  A.  Davis  that  a  subsidence  of  the  sea-bottom  is  necessary 
for  the  development  or  maintenance  of  offshore  bars.  As  this 
conclusion  is  of  much  importance  in  connection  with  the  problem 
of  recent  coastal  subsidence,  we  will  return  to  it  in  a  later  para- 
graph. That  portion  of  Shaler's  statement  which  calls  for  offshore 
deposition  of  wave-eroded  debris  before  bar  formation  can  begin. 


YOUNG  STAGE  855 

seems  unnecessary;  for  simple  uplift  of  a  very  gently  sloping  sea- 
bottom  will  produce  the  shallow  offshore  bottom  which  alone  is 
necessary  for  the  application  of  the  theory  of  bar  formation  which 
he  supports.  The  opinion  that  tidal  inlets  are  really  outlets 
formed  by  land  waters  bursting  through  a  formerly  more  or  less 
continuous  bar,  an  opinion  expressed  by  others-"  besides  Shaler, 
is  not  supported  by  the  evidence.  Inlets  are  continually  being 
opened  through  offshore  bars  and  through  bay  bars  which  are 
already  so  discontinuous  as  to  make  the  damming  of  land  water 
an  impossibility.  The  forcing  of  the  opening  from  the  seaward 
side  by  wave  attack  has  repeatedly  been  observed;  and  the 
sudden  rise  of  water  in  the  lagoon  immediately  after  the  breach- 
ing of  the  bar,  as  at  Scituate  during  the  storm  of  1898,  proves 
that  the  sea,  and  not  the  land  waters  in  the  lagoon,  may  be  the 
higher.  Occasional  inlets  may  be  opened  from  the  landward 
side;  but  as  a  rule  the  beach  is  forced  by  the  waves  of  the  sea. 
The  theories  of  de  Beaumont  and  Gilbert  seem  most  worthy 
of  critical  consideration.  It  does  not  seem  necessary  to  rely 
upon  ordinary  "  shore  drift  "  either  for  the  initiation  or  main- 
tenance of  offshore  bars,  as  is  required  by  Gilbert's  theory. 
There  is,  to  be  sure,  abundant  evidence  of  longshore  transporta- 
tion of  debris  on  the  seaward  side  of  most  offshore  bars;  but  it 
seems  impossible  to  assign  the  vast  volumes  of  material  in  the 
great  bars  along  the  south  Atlantic  and  Gulf  coasts  of  the  United 
States  to  a  source  at  one  or  the  other  end  of  such  bars  where 
they  may  connect  with  the  mainland,  or  may  recently  have  done 
so.  The  supply  of  debris  from  headlands  is  so  small,  and  the 
loss  of  material  from  attrition  under  wave  action  along  the 
face  of  the  bars  must  in  the  aggregate  be  so  large,  that  notwith- 
standing the  impossibility  of  making  a  reliable  comparison  be- 
tween these  two  factors,  one  is  impressed  with  the  probability 
that  the  bars  would  suffer  rapid  destruction  were  some  other 
source  of  supply  not  available.  An  adequate  source,  both  for 
the  initial  building  of  the  bars  and  for  their  maintenance  during 
a  slow  landward  migration,  is  furnished  by  the  shallow  sea- 
bottom;  and  the  on-  and  offshore  action  of  waves  is  alone 
sufficient  to  excavate  this  material  and  build  it  into  bars.  That 
some  material  is  also  furnished  by  longshore  transportation 
from  the  bases  of  cliffed  headlands,  and  that  material  eroded 
from  the  sea-bottom  suffers  longshore  transportation,  is  not  to 


356  DEVELOPMENT  OF  THE  SHORELINE 

be  doubted.  Such  action  must,  however,  be  regarded  as  inci- 
dental and  not  vital  to  offshore  bar  formation. 

Deductive  Study  of  Offshore  Bar  Profiles.  —  The  fact  that  Gil- 
bert's theory  of  offshore  bar  formation  does  not  imply  erosion 
of  the  sloping  sea  floor,  whereas  de  Beaumont's  theory  requii'es 
such  erosion,  suggests  that  the  difference  in  profiles  expectable  in 
the  two  cases  might  enable  one  to  determine  which  of  these  two 
most  promising  theories  is  best  able  to  explain  existing  offshore 
bars.  In  other  words,  it  occurred  to  the  writer  that  the  actual 
profiles  of  present-day  offshore  bars  should  clearly  indicate  the 
effects  of  extensive  bottom  erosion  if  de  Beaumont's  theory  be 
correct,  whereas  such  pronounced  evidence  of  bottom  erosion 
should  be  lacking  if  the  bars  formed  according  to  Gilbert's 
theory.  I  therefore  suggested  this  problem  to  Miss  Bertha  M. 
Merrill,  a  graduate  student  in  physiography  at  Columbia  Uni- 
versity, as  one  which  might  yield  tangible  results.  In  the  follow- 
ing paragraphs  I  have,  with  her  permission,  drawn  freely  upon 
her  report  of  profile  studies. 

It  may  be  noted  that  de  Beaumont's  theory  does  not  ex- 
clude the  possibilit}^  of  some  longshore  transportation  of  debris 
by  current  action,  although  it  necessarily  implies  that  such 
action  must  be  of  minor  importance.  Debris  cut  from  the 
original  sea-bottom  is  sufficient  to  form  the  bar,  and  is  assumed 
to  be  the  principal  source  of  suppl3^  Gilbert's  theory  would 
seem  on  first  reading  to  exclude  all  erosive  action  of  onshore 
waves;  but  it  is  doubtful  whether  that  author  would  altogether 
deny  a  minor  role  to  debris  eroded  from  the  sea-bottom  by  the 
waves,  and  by  them  contributed  to  the  growing  bar.  The 
essence  of  Gilbert's  theory  is  that  the  bar  absolutely  depends 
for  its  existence  upon,  and  is  therefore  largely  composed  of, 
debris  brought  from  a  distance  by  longshore  currents.  It  would 
appear,  therefore,  that  the  profiles  established  by  either  of  the 
two  methods  of  bar  formation  operating  alone  might  be  slightly 
modified  by  the  minor  co-operation  of  the  other  method;  but 
that  such  modifications  would  be  so  slight  as  not  materialh'  to 
change  the  essential  nature  of  the  profile  characteristic  of  each 
method. 

It  will  be  convenient  to  consider  first  the  profiles  expectable 
on  the  basis  of  Gilbert's  theory.  Figure  90  shows  the  profile 
of  a  partially  emerged  coastal  plain  near  the  shore  of  which  a 


YOUNG  STAGE 


357 


bar  (&)  has  been  built  upon  the  uneroded  sea-bottom  through 
deposition  by  longshore  currents.  Because  the  bottom  has  not 
been  eroded,  the  projection  of  the  sea-bottom  slope  {ss')  will  in- 
tersect the  sealevel  surface  at  the  inner  edge  of  the  lagoon  (l). 
Even  if  the  land  area  be  dissected  subsequent  to  uplift,  the  pro- 


FiG.  90. 


jection  of  the  sea-bottom  slope  will  still  intersect  the  sealevel 
surface  at  the  inner  edge  of  the  lagoon,  although  it  will  no  longer 
coincide,  as  in  the  initial  stage,  with  the  land  surface. 

In  case  the  sea-bottom  is  aggraded  in  the  vicinity  of  the  bar, 
but  decreasingly  so  seaward  from  the  bar,  the  projection  of  the 


Fig.  91. 

aggraded  sea-bottom  slope  (.s.s',  Fig.  91)  will  intersect  the  sea- 
level surface  seaward  from  the  inner  edge  of  the  lagoon. 

We  may  imagine  a  third  case  in  which  the  sea-bottom  is 
aggraded  in  the  vicinity  of  the  bar,  but  to  an  increasing  extent 
as  one  goes  seaward.     Then  the  projection  of  the  aggraded  sea- 


bottom  slope  (ss',  Fig.  92)  would  intersect  the  sealevel  surface 
landward  from  the  inner  edge  of  the  lagoon.  This  case  is  highly 
improbable,  for,  according  to  Gilbert's  theory,  the  bar  is  built 
up  in  the  zone  of  maximum  wave  agitation.  This  zone  occurs 
where  the  greatest  number  of  waves  expend  their  maximum 
energy  upon  the  sea-bottom.  Seaward  from  the  bar,  agitation 
is  less  because  fewer  waves  are  large  enough  to  brea-k  there. 


358 


DEVELOPMENT  OF  THE  SHORELINE 


Since  deposition  is  dependent  upon,  and  proportional  to,  the 
amount  of  agitation,  deposition  decreases  gradually  away  from 
the  bar.  Hence  it  is  difficult  to  conceive  an'area  of  maximum 
deposition  at  h,  an  area  of  little  or  no  deposition  at  s,  and  an 
area  of  increasing  deposition  at  s'. 

We  conclude  that  in  all  profiles  expectable  according  to  the 
Gilbert  theory,  the  sea-bottom  slope  projected  will  intersect  the 
sealevel  surface  at  or  seaward  from  the  inner  margin  of  the  lagoon. 

Let  us  next  consider  the  profiles  which  might  characterize 
offshore  bars  constructed  according  to  the  de  Beaumont  theory. 
Figure  93  shows  such  a  profile  in  which  the  original  slope  of 
a  partially  emerged  coastal  plain  {cc')  has  been  eroded  by  the 


Fig.  93. 


waves  to  produce  a  new  sea-bottom  (ss'),  while  a  portion  of  the 
debris  has  been  thrown  up  into  an  offshore  bar  (6).  It  appears 
that  the  projection  of  the  sea-bottom  slope  {ss')  will  intersect 
the  sealevel  surface  some  distance  landward  from  the  inner 
margin  of  the  lagoon.  Such  a  case  would  occur  when  all  the 
material  cut  from  the  sea-bottom  was  either  piled  .up  in  the  bar 


Fig.  94. 

or  carried  too  far  seaward  to  affect  this  portion  of  the  profile. 
A  more  probable  profile  is  that  represented  in  Figure  94,  which 
shows  the  sea-bottom  aggraded  by  deposition  of  part  of  the 
erosion  products  (sO  seaward  from  the  zone  of  maximum  wave 
attack  (s).  Again  the  projection  of  the  sea-bottom  slope  {ss') 
will  intersect  the  sealevel  surface  some  distance  landward  from 
the  inner  margin  of  the  lagoon. 


YOUNG  STAGE 


359 


In  both  the  above  cases  we  have  imagined  that  the  angle  of 
slope  of  the  initial  coastal  plain  and  its  seaward  continuation  is 
greater  than  the  angle  of  slope  of  the  newly  fashioned  sea- 
bottom.  It  is  conceivable,  however,  that  the  original  slope  of 
the  coastal  plain  might  be  so  extremely  gentle  that  the  new 
submarine  slope  would  be  appreciably  steeper.  Such  a  condi- 
tion is  represented  in  Figure  95,  from  which  it  will  be  seen  that  in 
cases  of  this  kind  the  projection  of  the  sea-bottom  slope  (.s.s') 


Fig.  95. 

may  intersect  the  sea-level  surface  at  or  seaward  from  the  inner 
margin  of  the  lagoon.  The  situation  would  be  the  same,  of 
course,  were  the  more  steeply  sloping  sea-bottom  a  surface  of 
aggradation,  as  shown  in  Figure  96.  It  would  seldom  happen 
that  the  projected  sea-bottom  slope  would  emerge  exactly  at 


Fig.  96. 


the  inner  margin  of  the  lagoon.  It  should  be  noted  that  in 
cases  of  this  kind  the  very  gentle  initial  offshore  slope  will 
cause  waves  to  break  far  from  land,  and  the  resulting  bar  will 
enclose  a  lagoon  of  exceptional  width. 

We  conclude,  therefore,  that  in  profiles  expectable  according 
to  the  de  Beaumont  theory  the  sea-bottom  slope  'projected  will 
intersect  the  sealevel  surface  landward  from  the  inner  margin  of 
the  lagoon,  except  in  those  cases  where  the  original  surface 
slope  is  exceptionally  low. 

We  may  summarize  the  results  of  the  deductive  study  of 
profiles  as  follows: 


360  DEVELOPMENT  OF  THE  SHORELINE 

Class  I.  If  the  sea-bottom  slope  projected  intersects  the 
sealevel  surface  at  the  inner  margin  of  the  lagoon,  the  offshore 
bar  was  probably  formed  according  to  Gilbert's  theory. 
[:  Class  II.  If  the  sea-bottom  slope  projected  intersects  the 
sealevel  surface  landward  from  the  inner  margin  of  the  lagoon, 
the  bar  was  probably  formed  according  to  de  Beaumont's 
theory. 

Class  III.  If  the  sea-bottom  slope  projected  intersects  the 
sealevel  surface  seaward  from  the  inner  margin  of  the  lagoon, 
the  bar  may  have  been  formed  according  to  either  theory; 
where  the  seaward  slope  of  the  land  is  at  all  pronounced,  prob- 
abilities- favor  the  Gilbert  theory;  where  the  coast  is  unusually 
flat  and  the  lagoons  very  broad,  the  de  Beaumont  theory  may  apply. 

Comparison  of  Actual  Profiles  of  Offshore  Bars.  —  To  test  the 
merits  of  the  two  theories,  eighteen  profiles  were  construjted  for 
coasts  having  well-developed  offshore  bars.  For  this  purpose  the 
United  States  Coast  and  Geodetic  Survey  charts  and  the  United 
States  and  Dutch  Hydrographic  charts  were  used.  In  order  to 
eliminate  from  the  profiles  local  and  minor  irregularities  of  the 
submarine  slope,  all  the  soundings  within  a  zone  of  certain  width, 
varying  from  five  to  seven  miles  according  to  circumstances, 
were  projected  on  a  single  vertical  plane  normal  to  the  shore- 
line, and  the  mean  curve  taken  as  the  profile  for  that  zone.  Be- 
cause such  bars  appear  in  great  perfection  off  our  own  Atlantic 
and  Gulf  coasts,  and  because  these  coasts  have  been  thoroughly 
charted,  a  majority  of  the  profiles  were  taken  from  these  regions. 
The  others  were  constructed  across  bars  of  the  North  Holland, 
German,  and  Venetian  coasts. 

The  results  for  each  case,  with  appropriate  comments,  are 
briefly  presented  below: 

Figure  97,  Profile  through  the  Gulf  of  Venice.  From  United 
States  Hydrographic  Chart,  Adriatic  Sheet  I.  The  sea-bottom 
slope  projected  (broken  line)  intersects  the  sealevel  surface  land- 
ward from  the  inner  margin  of  the  lagoon,  thus  placing  the 
profile  in  Class  II. 

Figure  98,  Profile  through  the  Kurische  Nehrung  and  Half 
on  the  Baltic  coast.  From  United  States  Hydrographic  Chart, 
Baltic  Sheet  II.  The  sea-bottom  slope  projected  again  inter- 
sects the  sealevel  surface  landward  from  the  inner  margin  of 
the  Haff,  showing  that  this  profile  also  belongs  in  Class  II. 


YOUNG  STAGE 


361 


fa 


362  DEVELOPMENT  OF  THE   SHORELINE 

Figures  99,  100.  101,  Profiles  through  the  TerscheUing,  Ameland, 
and  Vlieland  bars  on  the  North  Holland  coast.  From  Dutch 
Hydrographic  Charts  Xos.  205  and  224.  The  profile  through 
the  TerscheUing  bar  clearly  belongs  in  Class  II.  In  the  case  of 
the  Ameland  and  Vlieland  bars,  too  little  of  the  sea-bottom  slope 
is  shown  on  the  chart  to  serve  as  a  basis  for  projection;  but 
from  the  relation  of  these  areas  to  the  TerscheUing  area,  and 
from  other  data  for  the  sea-floor  topography,  it  is  known  that 
both  profiles  belong  in  Class  II. 

Figures  102,  103,  Profiles  through  the  Cape  Hatteras  bar.  North 
Carolina  coast.  From  United  States  Coast  Survey  Charts  Nos. 
1232  and  1229.  The  coast  is  "  an  excessively  flat  plain,"  and 
the  lagoon  exceptionally  wide.     Both  profiles  belong  in  Class  III. 

Figure  104,  Profile  through  Currituck  Beach,  northern  coast  of 
North  Carolina.  From  United  States  Coast  Survey  Chart  No. 
1229".  This  part  of  the  coast  is  less  flat  and  the  lagoons  corre- 
spondingly narrower  than  further  south  where  the  profiles  shown 
in  Figures  102  and  103  were  taken.  The  profile  through  Curri- 
tuck Beach  unquestionably  belongs  to  Class  II,  as  shown  by 
Figure  104. 

Figure  105,  Profile  through  Assateague  Island  bar,  Maryland. 
From  United  States  Coast  Survey  Chart  No.  1220.  The  profile 
appears  to  show  local  submarine  bars,  possibly  of  the  low  and 
baU  type  discussed  later,  and  clearly  belongs  to  Class  II. 

Figures  106,  107,  Profiles  through  the  offshore  bar  of  the  New 
Jersey  coast,  near  Barnegat  Inlet.  From  United  States  Coast 
Survey  Charts  Nos.  121  and  122.  Both  profiles  belong  in 
Class  II. 

Figure  108,  Profile  through  Fire  Island  bar,  south  coast  of 
Long  Island,  New  York.  From  United  States  Coast  Survey 
Chart  No.  1214.     The  profile  belongs  in  Class  II. 

Figure  109,  Profile  through  Galveston  Bay  and  Bolivar  bar, 
Texas  coast.  From  United  States  Coast  Survey  Chart  No. 
204.     The  profile  belongs  in  Class  II. 

Figure  110,  Profile  through  Matagorda  Bay  and  bar,  Texas 
coast.  From  United  States  Coast  Survey  Chart  No.  207.  The 
profile  belongs  in  Class  II. 

Figures  111,  112, 113,  and  114.  Profiles  through  Laguna  Madre 
and  Padre  Island  bar,  Texas  coast,  in  latitudes  27°  25',  26°  10', 
26°  45'   and  26°  25'   respectively.     From   United   States   Coast 


YOUXG  STAGE 


363 


id 


364 


DEVELOPMENT  OF  THE  SHORELINE 


PH 


f^ 


YOUNG   STAGE  365 

Survey  Charts  Nos.  210,  211,  and  212  respectively.  The  pro- 
files are  arranged  according  to  increasing  breadth  of  lagoon. 
The  first  three  clearly  fall  in  Class  II;  and  the  fourth,  where 
the  lagoon  is  exceptionally  broad,  appears  to  do  so,  although  it 
closely  approaches  the  conditions  of  Class  I.   • 

Summarizing  the  results  obtained  from  the  foregoing  exam- 
ination of  profiles  through  offshore  bars,  we  note  that  out  of 
eighteen  profiles  studied,  sixteen  fall  in  Class  II,  although  one 
of  these  approaches  closely  the  conditions  of  Class  I.  The  two 
remaining  profiles  fall  in  Class  III.  Both  the  two  profiles  of 
Class  III  and  the  profile  closely  approaching  Class  I  occur  off 
very  flat  coasts  where  the  lagoons  are  exceptionally  wide,  as 
would  be  expected  were  the  bars  formed  according  to  the  theory 
of  de  Beaumont.  In  other  words,  fifteen  of  the  profiles  cer- 
tainly fall  in  Class  II,  indicating  that  the  bars  were  formed 
according  to  the  de  Beaumont  theory;  while  the  remaining 
three  profiles,  explicable  according  to  either  the  Gilbert  or  the 
de  Beaumont  theory,  show  features  suggesting  that  they  also 
were  formed  according  to  the  de  Beaumont  theory. 

It  might  be  argued  that  the  bars  first  formed  according  to 
the  Gilbert  theory  and  were  then  pushed  landward,  the  waves 
cutting  into  the  sea-bottom  and  adding  part  of  the  erosion 
products  to  the  bars.  This  would  be  to  assume  an  initial  stage 
of  bar  formation  the  validity  of  which  could  not  be  tested  by 
appropriate  facts  of  observation,  and  to  admit  that  the  bars  as 
we  now  see  them  owe  their  existence,  in  part  at  least,  to  the 
process  outlined  by  de  Beaumont  and  more  fully  described  by 
Davis,  Under  these  circumstances  it  is  perhaps  more  reasona- 
ble to  accept  the  de  Beaumont  theory  of  bar  formation,  not  for- 
getting, however,  that  longshore  transportation  of  debris  is  an 
accessory  process  of  ver^^  great  importance. 

'  Development  of  the  Offshore  Bar.  —  In  tracing  the  development 
of  an  offshore  bar  we  may  therefore  imagine  a  gradually  shallow- 
ing sea-bottom  on  which  small  waves  break  at  the  initial  shore- 
line and  excavate  a  marine  cliff  and  bench,  while  large  waves 
break  farther  out  and  proceed  to  excavate  the  same  forms  in  the 
offshore  bottom.  Along  the  outer  zone  part  of  the  excavated 
material  is  deposited  just  landward  of  the  breakers,  in  less  agi- 
tated water;  that  is,  on  the  crest  of  the  submarine  cliff.  As  the 
waves  excavate  deeper  and  farther  landward  the  deposit  on  the 


366  DEVELOPMENT  OF  THE   SHORELINE 

summit  of  the  submarine  cliff  increases  in  volume  until  a  sub-^ 
marine  bar  of  significant  height,  and  indefinite  length  parallel 
to  the  inner  shoreline,  is  formed.  Further  growth  brings  the 
crest  of  the  bar  above  water  at  irregular  intervals,  giving  a  chain 
of  islands  separated  by  wide  spaces  of  shallow  water  covering 
the  still  submerged  portions  of  the  crest.  With  continued  exca- 
vation along  the  seaward  face  of  the  bar  and  addition  to  its  crest, 
the  islands  increase  in  number  and  in  length,  progressively  nar- 
rowing the  water  spaces  between  them  and  ultimately  coalescing 
to  a  greater  or  less  extent  to  form  a  more  nearly  complete  barrier 
between  the  open  sea  and  the  shallow  lagoon. 
,  Tidal  waters  which  formerly  ebbed  and  flowed  across  the 
wholly  submerged  bar  with  little  hindrance,  now  find  themselves 
confined  to  a  limited  number  of  increasingly  narrower  passage- 
ways between  the  ever  lengthening  above-water  portions  of  the 
bar.  As  the  openings  decrease  in  size,  the  tidal  currents  (in- 
cluding the  all-important  hydraulic  currents  generated  by  tidal 
action)  flowing  into  and  out  of  the  lagoon  increase  in  velocity. 
They  compensate  in  some  measure  for  the  increasingly  restricted 
breadth  of  their  passageways  by  cutting  deeper  channels  across 
the  still  submerged  portions  of  the  bar;  and  it  seems  probable 
that  this  process  may  often  be  carried  so  far  that  tidal  channels 
are  cut  clear  through  the  bar  and  into  the  original  sea-bottom 
below. 

As  the  submarine  bar  approaches  the  surface  it  comes  more 
and  more  under  the  influence  of  the  local  wind-generated  waves 
which  affect  the  water  to  a  shallow  depth  only.  As  a  majority 
of  these  waves  strike  the  seaward  face  of  the  bar  obliquely, 
beach  drifting  alongshore  becomes  increasingly  important,  and 
soon  is  the  dominant  factor  in  the  narrowing  of  tidal  inlets. 
No  longer  are  the  above-water  portions  of  the  bar  extended  and 
the  inlets  narrowed  mainly  by  simple  vertical  upbuilding  of  the 
still  submerged  parts  of  the  bar.  Instead,  the  debris  eroded 
from  the  bottom  and  cast  up  against  the  face  of  the  bar  is  at- 
tacked by  oblique  wind-made  waves  and  transported  laterally 
to  be  deposited  at  the  ends  of  the  elongated  islands,  thereby 
increasing  their  length  and  narrowing  the  inlets.  This  action 
is  directly  opposed  to  that  of  the  tidal  currents  which  pass  in 
and  out  of  the  inlets  and  endeavor  to  keep  them  open  by  re- 
moving material  deposited  by  the  longshore  currents.     So  long 


YOUNG   STAGE  367 

as  the  longshore  action  is  dominant,  the  inlets  continue  to 
narrow;  but  this  very  narrowing,  by  confining  the  tidal  currents 
to  smaller  and  smaller  cross  sections,  progressively  increases 
their  velocity.  A  time  must  come  when  the  inlets  are  narrowed 
enough  to  give  the  tidal  currents  a  strength  equivalent  to  that 
of  the  longshore  currents.  Thereafter  deposition  at  the  margins 
of  the  inlets  by  longshore  currents  is  followed  by  equivalent 
erosion  through  the  agency  of  tidal  currents.  Equilibrium  be- 
tween the  two  opposing  forces  is  established,  and  the  breadth  of 
the  inlets  remains  approximately  constant. 

The  required  breadth  may  be  maintained  by  a  few  compara- 
tively broad  inlets,  or  a  larger  number  of  narrower  inlets.  Since 
a  larger  tidal  range  means  stronger  tidal  currents,  we  should 
expect  to  find  some  relation  between  the  range  of  the  tide  along 
a  given  coast  and  the  number  or  size  of  the  inlets  through  its 
offshore  bars.  Such  a  relation  seems  to  exist.  Thus  along  the 
New  Jersey  coast,  where  the  tidal  range  is  from  4  to  5  feet,  in- 
lets are  more  frequent  than  along  the  coast  of  Texas  where 
with  a  tidal  range  of  but  1  or  2  feet  one  offshore  bar  extends  un- 
broken for  about  100  miles. 

Factors  Controlling  the  Number  and  Breadth  of  Tidal  Inlets.  — 
It  is  commonly  assumed  that  the  amplitude  of  the  tide  is 
the  only  factor  involved  in  determining  the  number  and  width 
of  tidal  inlets  through  offshore  bars.  Both  theoretical  con- 
siderations and  field  observations  negative  this  assumption.  In 
addition  to  the  varying  strength  of  longshore  action  (mainly 
beach  drifting),  the  volume  of  land  water,  the  extent  to  which  the 
lagoon  is  filled  with  sediment  or  marsh  deposits,  the  abundance 
and  rapidity  with  which  debris  is  supplied,  and  the  strength  of 
storm-wave  attack,  are  all  factors  o  importance.  With  the 
same  tidal  range  along  two  offshore  bars,  it  may  happen  that 
longshore  current  action  is  weak  on  one,  but  vigorous  on  the 
other.  Under  such  conditions  the  one  with  the  weaker  long- 
shore currents  will  have  more  or  wider  inlets.  Where  large 
rivers  empty  into  a  lagoon,  the  ebb  current  of  the  tide  is  greatly 
reinforced  by  the  land  waters,  and  will  keep  open  inlets  which 
would  otherwise  l)e  narrowed  or  closed.  As  sedimentation  and 
marsh  growth  decrease  the  water  space  of  the  lagoon,  the  volume 
of  tidal  waters  admitted  and  the  strength  of  the  tidal  currents  is 
reduced,  in  consequence  of  which  longshore  currents  may  be 


368        DEVELOPMENT  OF  THE  SHORELINE 

able  to  narrow  or  even  close  some  of  the  inlets.  If  an  abun- 
dance of  debris  is  supplied  to  longshore  currents  with  great 
rapidity,  the  closing  of  inlets  will  be  more  readily  accomplished 
than  if  a  smaller  amount  of  debris  is  supplied  very  slowly.  An 
inlet,  once  closed,  might  never  be  re-opened  were  it  not  for 
breaches  made  in  the  bar  by  storm-wave  attack.  Tidal  action 
tends  to  keep  inlets  open;  but,  except  in  the  case  of  an  unusually 
high  tide  overflowing  a  low  point  on  a  bar,  does  not  tend  to  pro- 
duce inlets.  Impounded  land  water  may  in  rare  instances  open 
an  inlet  after  the  manner  described  by  Shaler;  but  inlets  are 
more  commonly  re-opened  during  exceptional  storms  by  vigor- 
ous wave  erosion.  A  bar  exposed  to  the  waves  of  an  occasional 
great  storm  may  thus  be  breached,  where  one  less  exposed  would 
remain  intact. 
X  On  the  other  hand,  it  matters  little  how  many  inlets  may  be 
opened  by  the  waves,  longshore  currents  will  soon  close  all 
except  those  kept  open  by  tidal  currents  reinforced  by  outflow- 
ing land  waters.  If  the  tidal  range  is  such  as  to  generate  currents 
capable  of  maintaining  two  inlets  of  a  given  breadth  through  a 
certain  bar,  and  storm  waves  cut  two  additional  inlets,  the  tidal 
waters  will  for  a  time  flow  through  the  greater  number  of  open- 
ings with  decreased  velocities.  Longshore  currents  will  therefore 
dominate  the  tidal  currents  at  the  inlets,  until  deposition  has 
narrowed  all  of  the  inlets,  or  closed  two  of  them  (often  the  older 
ones),  leaving  the  other  two  of  the  required  breadth  and  thereby 
re-establishing  a  condition  of  equilibrium.  Or,  if  a  storm  drives 
waves  obliquely  upon  a  coast  in  such  manner  as  greatly  to  accel- 
erate the  longshore  transportation  of  debris,  all  the  inlets  through 
a  bar  may  be  closed  by  excessive  deposition  in  spite  of  tidal  cur- 
rents Once  the  inlets  are  closed,  the  tidal  currents  cease  to 
exist;  and  the  inlets  will  remain  closed  imtil  storm  waves  or  some 
other  agency  makes  new  breaches  through  the  bar.  n\  general 
we  may  say  that  waves  tend  to  make  inlets,  tidal  currents  to 
preserve  them,  and  longshore  currents  to  close  them. 

Theory  of  Tidal  Inlet  Distribution.  —  That  the  supply  of  debris 
brought  by  longshore  currents  may  be  more  important  than 
differences  of  tidal  range  in  determining  the  number  of  inlets 
through  a  bar,  is  apparent  from  a  study  of  certain  offshore  bars 
which  are  supplied  with  debris  derived  from  headlands  to  which 
the  bar  is  at  one  end  attached.     Let  us  deduce  the  conditions 


YOUNG   STAGE  369 

which  theoretically  should  characterize  offshore  bar  and  lagoon 
development  when  the  bar  is  attached  to  a  headland,  and  long- 
shore currents  move  from  the  headland  toward  the  further  ex- 
tremity of  the  bar. 

In  the  first  place,  it  is  evident  that  while  wave  currents  may 
remove  much  material  from  the  face  of  the  bar  and  transport  it 
seaward  to  deeper  water,  near  the  headland  the  loss  may  be 
more  or  less  completely  made  good  by  new  debris  brought  from 
the  adjacent  source  of  supply  by  longshore  currents.  The  effect 
of  this  accession  of  debris  is  two-fold:  the  bar  withstands  the 
normal  tendency  of  the  waves  to  drive  it  landward  since  the 
waves  have  all  they  can  do  to  take  care  of  the  new  material  con- 
tinually being  added  to  its  face;  and  for  the  same  reason  the 
waves  are  less  apt  to  cut  inlets  through  the  bar,  while  longshore 
currents  utilize  the  abundant  debris  to  seal  up  such  inlets  as 
may  occasionally  be  formed.  Accordingly  we  should  expect  a 
tendency  for  lagoons  to  be  broad  and  bars  to  be  continuous  in 
the  vicinity  of  headlands. 

Toward  that  end  of  the  bar  most  remote  from  the  headland, 
conditions  are  very  different.  The  debris  from  the  headland 
has  been  ground  fine  in  the  course  of  its  journey,  and  largel}'' 
dissipated.  Wave  attack  expends  its  full  energy  upon  a  bar 
which  receives  little  material  from  the  distant  headland  to  offset 
the  ravages  of  marine  erosion.  Hence  the  bar  is  driven  land- 
ward with  greater  ease,  and  during  its  retreat  the  waves  cut 
through  first  here,  then  there,  forming  inlets  which  are  not 
closed  as  readily  as  where  debris  is  more  abundantly  supplied. 
Far  from  headlands,  therefore,  there  should  be  a  tendency  for 
lagoons  to  be  narrow  and  for  bars  to  be  broken  by  frequent 
inlets. 

We  may  deduce  an  interesting  corollary  as  to  conditions 
within  the  lagoon.  Where  the  bar  is  continuous,  little  sediment 
from  its  seaward  side  can  reach  the  lagoon,  and  that  little  must 
be  brought  in  suspension  by  tidal  waters  entering  by  some 
distant  inlet.  Where  inlets  are  abundant,  more  sediment  can 
enter  the  lagoon  with  flood  tide,  even  though  this  be  the  part 
of  the  bar  most  poorly  supplied  with  debris  from  the  distant 
headland.  It  must  also  appear  that  the  end  of  the  lagoon  near 
the  headland  is  least  apt  to  have  a  constant  salinity.  At  times 
the  water  may  become  nearly  fresh,  while  high  tides  or  tempo- 


370  DEVELOPMENT  OF  THE   SHORELINE 

rary  inlets  will  result  in  a  high  salt  content.  Such  variations  in 
salinity  are  unfavorable  to  the  growth  of  either  marine  or  fresh 
water  vegetation.  On  the  other  hand,  where  numerous  inlets 
keep  the  lagoon  waters  constantly  salt,  marine  grasses  thrive 
and  contribute  effectively  to  the  filling  of  the  lagoon.  We 
conclude,  therefore,  that  theoretically  the  ''up-current"  or 
headland  end  of  a  lagoon  should  be  more  open  than  the  further 
end  where  marine  sediment  and  marine  vegetation  unite  to 
form  a  salt  marsh  filling. 

The  Theory  Tested.  —  If  we  turn  now  to  an  examination  of 
offshore  bars  and  lagoons  along  the  Atlantic  Coast,  we  find  that 
despite  the  manifest  possibility  of  other  factors  complicating  the 
situation,  there  exist  substantial  confirmations  of  the  theory  of 
inlet  formation  outlined  above.  (In  the  discussion  which  follows 
I  have  drawn  freely  upon  the  results  of  map  studies  made  by 
Miss  B.  M.  Merrill,  under  my  direction.)  On  the  south  side  of 
Long  Island  the  longshore  current  moves  westward  along  an 
offshore  bar  (Fig.  115)  which  is  attached  at  its  eastern  end  to  a 
complex  headland  consisting  of  a  terminal  moraine  and  outwash 
plain.  From  Southampton,  where  the  bar  really  springs  from 
the  mainland  (it  barely  touches  it  at  Quogue)  westward  to  the 
Gilgo  Lifesaving  Station,  a  distance  of  54  miles,  there  is  only 
one  inlet;  in  the  next  22  miles,  to  Far  Rockaway,  there  are 
three  inlets.  For  sake  of  easy  comparison  with  the  cases  which 
follow,  we  may  say  that  nearest  the  headland  the  inlets  occur  at 
the  rate  of  2  to  100  miles,  while  farther  away  the  rate  is  14  to 
100  miles.  Great  South  Bay,  the  main  lagoon,  is  wide  and 
comparatively  free  from  tide  marsh  in  the  half  nearest  the  head- 
land, narrower  and  almost  filled  with  marsh  in  the  farther  half 
where  inlets  are  frequent.  The  actual  conditions  are  precisely 
those  which  deduction  led  us  to  expect. 

The  New  Jersey  coast  is  fringed  by  an  offshore  bar  (Fig.  116) 
attached  at  its  northern  end  to  a  headland  consisting  of  the  cliffed 
coastal  plain  between  Long  Branch  and  Bayhead.  The  longshore 
current  moves  southward  from  the  headland.  In  the  first  50 
miles  there  are  2  inlets,  in  the  next  50  miles,  8  inlets.  In  other 
words,  nearest  the  headland  the  inlets  average  4  to  100  miles, 
farther  away  16  to  100  miles.  As  in  the  Long  Island  case  the 
half  of  the  lagoon  nearest  the  headland  has  the  greater  average 
width  and  the  smallest  amount  of  marsh  filling.     Toward  the 


Long  Branch 
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Page  371 


372  DEVELOPMENT  OF  THE  SHORELINE 

south,  where  inlets  are  frequent,  we  find  the  lagoon  narrow  and 
almost  completely  filled  with  marsh.  In  this  case  also  the 
observed  facts  conform  to  the  expectations  as  deduced  from  the 
proposed  theory  of  inlet  formation. 

Similar  conditions  obtain  off  the  coast  between  Delaware 
and  Chesapeake  Bays.  An  irregular  headland  extending  from 
Cape  Henlopen  to  Bethany  Beach  has  attached  to  it  and  ex- 
tending southward  an  offshore  bar  which  continues  for  some- 
thing over  50  miles  before  the  first  inlet  is  reached,  whereas  in 
the  next  50  miles  ten  inlets  occur.  The  relation  is  therefore 
roughly  expressed  by  assigning  a  rate  of  2  inlets  per  100  miles 
for  the  headland  end  of  the  bar,  and  20  inlets  per  100  miles  for 
the  farther  end.  Near  the  headland  end  we  have  the  open 
Chincoteague  Bay.  Farther  south  the  lagoon  is  first  narrow 
and  marsh-filled;  but  the  expectable  relations  are  then  masked 
by  a  widening  of  the  lagoon  area  possibly  as  the  result  of  an 
exceptionally  flat  initial  sea  floor  which  permitted  the  bar  to 
form  far  offshore.  It  should  be  noted,  however,  that  even 
here  the  inlets  are  close-spaced  and  the  lagoon  area  largely  filled 
by  marshes  or  mud-flats,  as  required  by  the  theory. 

The  Carolina  coast  is  so  complicated  by  the  three  cuspate 
bars  forming  Capes  Hatteras,  Lookout,  and  Fear  that  one  might 
scarcely  expect  to  find  the  relationships  characteristic  of  simple 
offshore  bars.  Yet  if  we  compare  different  sections  of  the  coast 
in  a  broad  way,  ignoring  local  abnormalities,  we  seem  to  see  the 
working  of  the  same  laws  controlling  cases  previously  discussed. 
The  headland  for  this  section  is  the  margin  of  the  coastal  plain 
of  Virginia,  south  of  Cape  Henry,  and  the  shore  currents  move 
in  a  general  north  to  south  direction.  We  may  recognize  four 
natural  subdivisions  of  the  coast:  a  first  section  from  the  head- 
land to  Cape  Hatteras,  a  second  between  Capes  Hatteras  and 
Lookout,  a  third  between  Capes  Lookout  and  Fear,  and  a 
fourth  between  Cape  Fear  and  a  point  just  west  of  Little  River, 
beyond  which  the  offshore  bar  seems  to  touch  the  mainland  again. 
In  the  first  section  the  inlets  number  but  2  in  a  distance  of 
113  miles,  and  the  lagoon  attains  a  great  width  with  compara- 
tively little  filling.  The  abnormal  width  in  parts  of  the  first 
two  sections  is  probal^ly  due  to  an  exceptionally  gentle  slope  of 
the  seafloor  along  the  Cape  Hatteras  axis.  In  the  second 
section  of  72  miles,  there  are  three  inlets,   giving  an  average 


YOUNG  STAGE  373 

spacing  of  4  to  100  miles,  and  the  lagoon  becomes  comparatively 
narrow  toward  Cape  Lookout.  In  the  third  section  the  number 
of  inlets  increases  to  9  in  100  miles,  while  the  lagoons  narrow 
still  more  and  become  much  more  filled  with  marsh  deposits. 
At  Cape  Fear  the  lagoon  broadens  out  considerably,  but  the 
width  here  is  only  seven  and  one-half  miles  as  compared  to 
twelve  and  one-half  at  Cape  Lookout,  or  thirty  miles  at  Cape 
Hatteras.  In  the  fourth  section  there  are  eight  inlets  in  40 
miles,  which  is  equivalent  to  a  spacing  of  20  inlets  to  100  miles; 
the  bar  is  driven  back  nearl}^  to  the  mainland,  and  the  narrow 
lagoon  is  almost  completely  filled  with  marsh.  Despite  its  com- 
plexities the  Carolina  case  appears  to  meet  the  requirements  of 
the  theory. 

The  Florida  offshore  bar  is  so  complicated  by  the  presence  of 
hard  coquina  along  some  of  its  parts,  by  the  complex  cuspate 
foreland  of  Cape  Canaveral,  and  by  coral  reefs  farther  south, 
that  it  does  not  properl}^  come  within  the  scope  of  our  enquiry. 
If  we  consider  the  Texas  coast,  however,  taking  the  Rio  Grande 
delta  as  the  headland  supplying  the  debris,  and  ignoring  the 
Rio  Grande  and  Brazos  Santiago  openings  in  the  immediate 
vicinity  of  the  delta,  we  find  an  offshore  bar  extending  north- 
ward, in  the  direction  of  what  appears  from  sand  migration  to 
be  the  dominant  longshore  current,  more  than  100  miles  before 
the  first  inlet  is  encountered.  Near  the  headland  we  have  the 
Laguna  Madre,  broadly  open,  but  shallow  because  of  the  very 
gentle  slope  of  the  initial  sea  floor.  Farther  north  the  lagoon 
proper  (no  account  should  be  taken  of  the  drowned  valley  bays) 
grows  narrower  and  the  proportion  of  marsh  filling  increases. 

In  all  of  the  cases  described  above  there  is  a  marked  tendency 
for  the  number  of  inlets  and  the  proportion  of  lagoon  filling  to 
increase,  and  for  the  width  of  lagoon  to  decrease,  with  increase 
of  distance  from  headlands.  This  seems  to  confirm  the  theory 
that  the  amount  of  debris  brought  from  headlands  by  longshore 
currents  exercises  an  important  control  over  the  number  of 
inlets  through  offshore  bars,  as  well  as  upon  the  rate  of  bar 
retreat  and  lagoon  filling.  It  should  be  noted,  however,  that 
in  each  case  the  tidal  range  increases  from  the  headland  toward 
the  farther  end  of  the  bar,  although  the  amount  of  increase 
between  sections  of  no  inlets  or  few  inlets,  and  sections  of  numer- 
ous mlets  is  sometimes  so  slight  as  to  be  of  doubtful  importance. 


374  DEVELOPMENT  OF  THE  SHORELINE 

Both  distance  from  headland  and  range  of  tide  co-operated  to 
produce  the  observed  results,  but  it  is  believed  the  former  factor 
is  the  more  important  of  the  two. 

Tidal  Deltas.  —  The  h3^draulic  currents  generated  by  the  tides, 
and  called  tidal  currents  in  the  foregoing  paragraphs  in  conformity 
with  well-nigh  universal  custom,  produce  certain  features  at  the 
inlets  which  deserve  brief  notice.  Debris  brought  by  beach  drift- 
ing or  other  longshore  currents  is  seized  by  the  inflowing  or  out- 
flowing current  at  the  inlet  and  transported  into  the  lagoon  or 
out  to  sea.  Most  of  the  debris  is  not  carried  far  before  being 
deposited  in  the  quieter  water  of  the  larger  water-body  to  form 
a  tidal  delta.-^  The  typical  tidal  delta  is. wholly  submerged  and 
is  double,  one  part  facing  landward  and  representing  the  result 
of  deposition  in  the  lagoon  by  incoming  currents;  the  other 
part  facing  seaward  and  owing  its  construction  to  deposition  in 
the  sea  by  outflowing  currents.  Because  the  seaward  part  of 
the  delta  is  exposed  to  the  action  of  waves  and  longshore  cur- 
rents it  is  commonly  stunted  in  its  growth  and  margined  by 
contours  of  simple  curvature;  only  that  portion  in  the  fagoon  is 
apt  to  acquire  appreciable  size  and  the  lobate  form  of  ordinary 
deltas  (Fig.  117). 

Migrating  Inlets.  —  The  exposure  of  most  beaches  to  wave 
attack  is  such  that  longshore  current  action  (usually  beach  drift- 
ing) in  one  direction  predominates  over  that  in  the  other.  This 
results  in  a  marked  tendency  for  inlets  to  migrate  in  a  certain 
definite  direction  — ■  that  of  the  dominant  current.  Deposition  on 
that  side  of  the  opening  to  which  longshore  currents  bring  abun- 
dant debris  tends  to  narrow  the  inlet,  whereas  erosion  alone  is 
operative  on  the  other  side.  An  excess  of  deposition  on  one  side, 
accompanied  by  erosion  alone  on  the  other,  must  result  in  a  lat- 
eral migration  of  the  inlet  along  the  hsu'  in  the  direction  of  the 
dominant  current,  while  the  breadth  of  the  inlet  remains  unim- 
paired. On  the  New  Jersey  coast  south  of  Barnegat  Inlet  the 
inlets  through  the  offshore  bar  migrate  southward,  while  north 
of  this  point  the  direction  of  inlet  migration  is  northward. 

The  presence  of  a  dominant  current  along  an  offshore  bar 
broken  by  inlets  results  in  the  development  of  offsets  and  over- 
laps similar  to  those  already  described  in  connection  with  bay 
bars  (Chapter  VI).  As  these  two  features  are  fully  explained  in 
the  connection  cited,  it  will  not  be  necessary  to  consider  them  at 


Page  375 


376 


DEVELOPMENT  OF  THE  SHORELINE 


greater  length  here.  We  may  simply  recall  in  passing  that  the 
southern  New  Jersey  coast  appears  to  afford  an  exception  to 
Gulliver's  rule^^  according  to  which  the  dominant  current  should 
"flow  from  the  outer  curve  toward  the  inner  one"  along  a  shore- 
line marked  by  offsets.  Here  the  direction  of  inlet  migration 
proves  that  the  dominant  current  is  from  the  northeast;  but 
according  to  Gulliver's  rule  the  offsets  at  the  inlets  north  of 
Cape  Maj^  would  require  a  current  from  the  southwest.  It  is 
clear  that  the  direction  of  offset  may  be  determined  in  certain 
cases  by  some  force  other  than  the  dominant  longshore  current. 
One  important  consequence  of  inlet  migration  which  seems 
not  to  have  been  fully  recognized,  will  claim  our  attention  when 


Fig.  118.  —  Stages  in  the  development  and  retrogression  of  an  ofTshore 
bar.     (After  Davis.) 


we  come  to  consider  the  landward  retrogression  of  the  offshore 
bar.  It  is  quite  generally  assumed  in  descriptions  of  this  last 
process  that  the  bar  comes  to  repose  wholly  upon  the  deposits 
of  the  lagoon  or  marsh  as  soon  as  it  has  moved  landward  a  dis- 
tance equivalent  to  its  own  breadth.  This  view  is  well  exempli- 
fied by  Figure  118,  reproduced  from  a  diagram  given  by  Davis 
in  his  "  Erklarende  Beschreibung  der  Landformen^'^ ".  It  is  quite 
evident,  however,  that  the  condition  represented  by  this  dia- 
gram could  only  obtain  where  inlet  migration  is  either  wholly 
absent  or  takes  place  slowly  at  the  same  time  that  bar  retro- 
gression is  comparatively  rapid.  For  the  migration  of  an  inlet 
along  the  bar  results  in  the  complete  removal  of  that   portion 


YOUNG  STAGE  377 

of  the  bar  and  its  underlying  deposits  toward  which  the  opening 
is  moving,  down  to  the  greatest  depth  reached  by  the  tidal 
channel;  while  deposition  on  the  up- currant  side  of  the  inlet 
forms  an  essentially  new  bar  whose  liase  rests,  not  on  the  sur- 
face of  the  lagoon  deposits,  but  upon  the  erosion  plane  formed 
by  the  lateral  migration  of  the  inlet.  Since  the  inlets  probably 
reach  to  or  below  the  original  shallow  sea-bottom  in  a  majority 
of  cases,  the  bar  to  leeward  of  the  migrating  inlet  will  commonly 
rest  on  the  original  sea-bottom  deposits. 

Inside  the  inlet  the  remains  of  the  tidal  delta  left  on  the 
up-current  or  leeward  side  of  the  opening  will  prolong  the  land- 
ward side  of  the  bar  into  the  lagoon  with  a  gentle  slope;  for  just 
as  successive  deposits  of  sand  on  the  up-current  side  of  the  inlet 
remain  to  form  the  new  part  of  the  bar,  so  the  side  of  the  delta 
away  from  which  the  inlet  is  migrating  is  progressively  left 
behind  to  form  a  sheet  of  sand  extending  from  the  new  bar  out 
into  the  lagoon  as  a  thinning  wedge.  If  the  offshore  bar  has  a 
marsh  behind  it  instead  of  aJi  open  lagoon,  the  result  is  essen- 
tially the  same.  Erosion  will  remove  both  the  bar  and  the 
adjacent  marsh  deposits  on  the  down-current  or  far  side  of  the 
inlet,  while  deposition  of  sand  on  the  up-current  or  near  side 
will  leave  a  bar  resting  on  the  eroded  sea-bottom.  This  bar 
will  be  extended  marshward  by  sand  deposited  along  the  near 
side  of  the  tidal  creek  connecting  with  the  inlet.  A  cross  sec- 
tion through  an  offshore  bar  and  marsh,  after  the  bar  had 
migrated  toward  the  land  a  great  distance,  would  in  this  case 
not  look  like  stage  4  in  Figure  118,  as  is  generally  assumed,  but 
more  like  stage  G  in  Figure  119. 

In  case -the  bar  moves  landward  an  appreciable  distance  after 
one  inlet  has  migrated  past  the  line  of  the  cross  section  and 
before  the  next  migrating  inlet  has  reached  that  line,  we  would 
have  the  conditions  represented  in  stage  F,  Figure  119,  where  the 
marsh  deposits  are  exposed  on  the  seaward  side  of  the  bar. 

It  is  eAddent  from  the  considerations  just  outlined  that  it  may 
be  difficult  or  impossible  to  determine  how  far  landward  from 
its  original  position  an  offshore  bar  has  migrated.  Were  the 
assumed  conditions  of  stage  4,  Figure  118,  commonly  present  after 
a  considerable  landward  migration,  the  problem  would  be  more 
simple;  for  soundmgs  made  through  the  marsh  deposits  would 
show  an  increasing  depth  of  these  deposits  until  the  margin  of 


tNip 


Submarine  Bar 
forming 


Offshore  Bar 


Marsh  Offshore  Bar 


Offshore  Bar 


Fig.  119.  —  Stages  in  the  normal  historj-  of  an  offshore  bar,  due  account- 
being  taken  of  the  effect  of  migrating  inlets.     Between  stages  F  and  G 
an  inlet  has  migrated  past  the  zone  of  the  cross  section,  producing  condi- 
tions similar  to  those  in  stage  C  or  D. 
Page  378 


YOUNG  STAGE  379 

the  superposed  bar  was  reached;  and  wells  drilled  on  the  bar 
would  pass  through  a  thick  layer  of  marsh  mud  under  the  beach 
sands.  On  the  other  hand,  soundings  showing  an  increasing 
thickness  of  marsh  deposits  for  some  distance  seaward  from  the 
inner  shoreline,  followed  by  a  gradually  decreasing  thickness  as 
the  bar  was  approached  (stage  2,  Fig.  118),  and  records  of  wells 
on  the  bar  showing  that  nothing  but  sand  was  encountered  by 
drilling,  would  indicate  that  the  bar  was  still  in  its  original  posi- 
tion. 

Unfortunately  such  reasoning,  although  frequently  followed, 
at  least  tacitly,  when  offshore  bars  are  discussed,  is  not  valid  if 
shifting  inlets  are  involved.  It  is  clear  from  Figure  119,  stage  G, 
that  the  results  of  soundings  and  the  well  records  accepted  above 
as  proving  no  landward  migration  of  the  bar,  would  be  obtaina- 
ble in  the  case  of  a  bar  which  had  really  migrated  far  from  its 
initial  position.  So  also,  soundings  or  well  records  might  indicate 
only  a  slight  landward  progress  of  the  bar,  whereas  the  actual 
movement  had  been  very  great.  Along  the  New  Jersey  coast, 
lines  of  soundings  across  the  marshes  show  that  beyond  the  axis 
of  the  marsh  the  peat  and  swamp  muds  thin  out  and  the  sandy 
bottom  rises  gradually  toward  the  offshore  bar.  Well  records 
frequently  show  that  no  marsh  deposits  were  encountered  in 
drilling,  or  that  only  small  thicknesses  of  such  deposits  were 
-found.  Since  the  bar  is  repeatedly  broken  through  by  shifting 
inlets  these  facts  cannot  be  regarded  as  evidence  that  the  bar 
has  changed  but  little  from  its  former  position,  any  more  than 
the  outcropping  of  small  quantities  of  peat  along  the  outer 
shorelines  can  be  accepted  as  proof  of  an  extensive  landward 
migration  of  the  bar.  Either  permanence  of  or  marked  change 
in  the  position  of  the  bar  must  be  proven,  if  at  all,  by  other 
lines  of  evidence. 

Lagoon  and  Marsh.  —  When  the  offshore  bar  is  formed  there 
is  enclosed  a  long,  narrow  lagoon  between  the  bar  and  the  inner 
shoreline,  the  lagoon  communicating  with  the  open  sea  by  means 
of  the  tidal  inlets.  Comparatively  quiet  water  in  the  lagoon 
favors  deposition  of  the  fine  debris  which  is  derived  from  three 
principal  sources.  The  products  of  attrition  resulting  from 
wave  action  on  the  outer  surface  of  the  bar  are  moved  to  the 
tidal  inlets  by  longshore  currents  and  the  finer  part  is  carried 
into  the  lagoon  by  tidal  currents,  to  be  widely  distributed  over 


380  DEVELOPMENT  OF  THE   SHORELINE 

the  shallow  bottom;  all  the  coarse  material  is  added  to  the  bar 
or  dropped  near  the  inlet  to  form  the  tidal  delta.  Rivers  may 
bring  sediment  from  the  land  surface  into  the  lagoon,  depositing 
the  coarser  part  in  the  form  of  deltas  along  the  inner  shoreline, 
and  delivering  the  finer  part  to  the  feeble  currents  in  the  lagoon 
for  wider  distribution.  Onshore  winds  blow  sand  from  the 
beach  and  dunes  of  the  bar  back  into  the  lagoon.  As  a  rule  the 
coarser  sand  quickly  drops  into  the  water  close  to  the  lagoon 
shore  of  the  bar,  and  only  the  finest  material  is  wafted  far  over 
the  surface  of  the  waters  before  dropping  into  them  to  find  a 
resting  place  on  the  submarine  floor. 

As  material  from  these  three  sources  accumulates,  the  bottom 
of  the  lagoon  is  built  upward  toward  the  surface.  If  the  supply 
of  fine  sediment  is  unusually  abundant  the  lagoon  may  eventu- 
ally become  filled  with  a  deposit  of  almost  pure  clay  or  sandy 
clay,  on  the  surface  of  which  grow  salt  marsh  grasses.  Prob- 
ably a  more  normal  histor}^  would  be  something  like  that  de- 
scribed by  Shaler^^  in  which  eel-grass  or  other  salt-water  plants 
first  gain  foothold  on  the  muddy  bottom  below  low-tide  level 
and  aid  the  process  of  deposition  by  checking  the  currents 
passing  through  them.  Later,  as  the  lagoon  bottom  reaches 
a  higher  level,  marsh  plants  are  able  to  colonize  the  surface,  and 
their  remains  may  form  no  inconsiderable  proportion  of  the 
completed  deposit.  The  entire  lagoon  is  thus  ultimately  filled 
with  a  clayey  formation  which  includes,  particularly  in  its  upper 
portions,  large  quantities  of  vegetable  matter;  while  its  surface 
is  covered  with  the  grasses  of  a  typical  growing  salt  marsh. 

Retrogression  of  Ojfshore  Bars.  — Just  as  continued  wave  attack 
ultimately  forces  the  recession  of  other  shoreline  features,  so  the 
offshore  bar  must  be  driven  landward  in  course  of  time.  As 
previously  explained  the  outer  shoreline  of  the  bar  may  tempo- 
rarily be  prograded;  local  disturbances  of  the  shore  profile  of 
equilibrium  may  cause  the  bar  to  widen  locally,  as  appears, to 
be  the  case  at  Atlantic  City;  or  general  and  long-continued 
excessive  supply  of  shore  debris  may  result  in  broadening  a  bar 
into  a  beach  plain  of  great  extent,  such  as  that  forming  Cape 
Canaveral  on  the  Florida  coast.  Occasionall}'  after  a  bar  is  built 
the  zone  of  bar  construction  is  shifted  so  rapidly  seaward  that  a 
broad  swale  or  lagoon  is  left  between  the  bar  earlier  formed  and 
its  later  counterpart.     If  the  swale  or  lagoon  be  occupied  by 


YOUNG  STAGE  381 

tnarsh,  the  first  bar  appears  as  a  long  ridge  of  dry  land  in  the 
midst  of  the  expanse  of  salt  grass  and  water  (Plate  XLIV).  But 
all  such  activities  are  temporary,  and  the  time  will  come  when 
loss  of  fine  material  from  attrition  and  removal  to  deep  water 
will  exceed  the  diminishing  supply  of  shore  debris.  The  waves, 
relieved  of  the  burden  of  excessive  debris  transportation,  will 
then  utilize  their  surplus  energy  in  eroding  the  sea-bottom  amd 
driving  the  bar  landward. 

Material  eroded  from  the  face  of  the  bar,  and  from  the  sea- 
bottom  below,  is  hurled  by  waves  to  the  bar  crest  or  even  be- 
yond, and  descends  the  black  slope  toward  the  lagoon  with  the 
assistance  of  over-wash  from  exceptionally  high  waves,  and 
running  water  due  to  rainfall.  This  insures  for  the  actively 
retreating  bar  a  narrow  breadth  and  an  asymmetrical  cross- 
profile,  the  front  slope  toward  the  sea  being  characteristically 
steeper  than  that  toward  the  lagoon;  while  the  lagoon  shore  is 
apt  to  show  a  series  of  rude  deltas  where  overwash  has  pro- 
jected beach  material  into  the  lagoon  waters.  If  the  lagoon  has 
been  replaced  by  salt  marsh,  the  features  are  essentially  the  same, 
except  that  the  overwash  deltas  spread  out  upon  the  marsh  sur- 
face (Plate  XLI),  while  the  marsh  muds  and  peat  may  become 
exposed  below  high  tide  on  the  seaward  side  of  the  bar. 

Migrating  tidal  inlets  tend  to  destroy  all  of  the  features  just 
mentioned:  the  asymmetry  of  the  bar  profile,  the  overwash 
deltas,  and  the  subjacent  relation  of  marsh  deposits  to  the  bar. 
If  the  bar  retreats  rapidly  while  inlets  are  few  and  migrate  slowly, 
the  features  described  may  be  observed,  except  along  that  por- 
tion of  the  bar  most  recently  re-formed.  If  the  bar  retreats 
slowly  and  intermittently,  while  inlets  are  numerous  and  migrate 
rapidly,  the  lack  of  symmetry  and  the  overwash  deltas  may  be 
poorly  developed,  while  marsh  deposits  beneath  the  bar  may  be 
nearly  or  entirely  lacking. 

Gulliver^^  considers  a  prograding  offshore  bar  as  character- 
istic of  the  youthful  stage  of  a  shoreline  of  emergence,  while  a 
retrograding  bar  is  the  distinguishing  feature  of  "  adolescence." 
The  exposure  of  marsh  deposits  on  the  seaward  side  of  the  bar  is 
necessarily  relied  upon  as  the  principal  proof  of  retrogression. 
Davis^^  defines  "  late  youth  "  as  the  period  when  the  bar  is 
driven  landward  far  enough  to  show  tide-marsh  turf  and  mud  on 
the  outer  side  of  the  bar.     One  must  doubt,  however,  whether 


382  DEVELOPMENT  OF  THE  SHORELINE 

it  is  feasible  to  utilize  such  criteria  as  a  basis  for  distinguishing 
different  stages  of  shoreUne  development.  In  the  first  place  it 
is  usually  impossible  to  tell  from  a  map  whether  an  offshore 
bar  is  advancing  or  retreating,  so  that  maps  would  be  of  little 
or  no  use  in  determining  whether  a  given  shoreline  was  in  youth 
or  in  its  adolescent  period  (late  youth).  This  difficulty  is  well  ex- 
emplified in  Gulliver's  essay,  where  several  shorelines  of  emer- 
gence are  arbitrarily  classified  as  "young,"  although  the  author 
admits  that  they  may  really  be  adolescent;  but  in  three  cases 
we  read  that  "  the  scale  of  the  map  is  too  small  to  show  indica- 
tions in  which  direction  the  bar  is  moving,"  "  whether  advanc- 
ing or  retreating  the  writer  does  not  know,"  and  "  the  writer 
could  find  no  evidence  as  to  which  way  it  is  moving." 

Even  if  field  observations  are  available  as  an  aid  to  classifi- 
cation, the  case  is  little  better.  An  offshore  bar  may  be  alter- 
nately retrograded  and  prograded  due  to  changing  conditions  of 
equilibrium  of  the  shore  profile.  It  will  hardly  help  us  to  assume 
that  such  a  shoreline  vibrates  from  youth  to  adolescence  and 
back  to  youth  again  repeatedly.  On  the  other  hand,  a  bar 
which  had  been  continuously  but  slowly  retrograded  for  a  long 
period  of  time  might  be  erroneously  assigned  to  the  youthful 
stage  in  case  migrating  inlets  removed  the  evidences  of  retro- 
gression most  commonly  depended  upon,  such  as  the  subjacent 
marsh  deposits. 

Normally  an  offshore  bar  should  never  prograde  to  any 
appreciable  extent,  but  should  retrograde  from  the  moment  of 
its  initiation.  Prograding  implies  a  disturbance  of  normal  con- 
ditions, a  variation  in  the  shore  profile  on  one  part  of  the  coast 
due  to  abnormal  activity  of  some  one  or  more  of  the  shore 
processes,  as  a  result  of  which  shore  debris  is  supplied  with 
exceptional  rapidity  to  that  part  of  the  shore  where  prograding 
takes  place.  A  possible  exception  to  this  statement  is  the  pre- 
sumably rare  case  in  which  progressively  larger  and  larger 
storm  waves  built  additions  to  the  initial  bar  farther  and  farther 
seaward.  It  seems  unwise  to  adopt  as  the  criterion  of  "  youth  " 
a  condition  which  has  no  sure  place  in  the  ideal  normal  history 
of  shoreline  development. 

The  three  considerations  set  forth  above  force  us  to  the  con- 
clusion that  no  great  profit  is  to  be  derived  from  the  attempt  to 
distinguish  different  stages  of  shoreline  development  according 


PROGRESSIVE   SUBSIDENCE  ON  LAGOON   HISTORY      383 

to  whether  the  offshore  bar  is  prograding  or  retrograding,  whereas 
considerable  confusion  and  misunderstanding  is  apt  to  result 
from  such  an  attempt.  We  will  therefore  regard  the  offshore 
bar,  with  its  associated  lagoon  or  marsh,  as  characteristic  of 
the  youthful  stage  of  the  shoreline  of  emergence,  making  no 
attempt,  in  the  present  state  of  our  knowledge  of  shorelines,  to 
further  subdivide  the  stage  of  youth.  On  this  basis  the  New 
Jersey  shoreline,  from  Bayhead  to  Cape  May  City,  is  a  young 
shoreline  of  emergence. 

Cuspate  Offshore  Bars.  —  Occasionally  an  offshore  bar  has  a 
pronounced  cuspate  pattern.  Such  is  the  case  at  the  Carolina 
Capes,  on  the  offshore  bar  bordering  North  Carolina.  Like 
ordinary  cuspate  bars,  the  cuspate  form  of  offshore  bars  may 
be  produced  in  a  variety  of  ways.  A  favorite  theory  is  that 
proposed  by  Abbe^^  for  the  Carolina  Capes,  and  supported  by 
Gulliver^^  and  Davis-^  according  to  which  the  cusps  result  from 
deposition  in  the  triangle  of  quieter  water  between  two  adjacent 
circling  currents.  A  shoal  or  a  former  island  some  distance  off 
a  straight  coast  not  infrequently  produces  a  cuspate  pattern  in 
adjacent  parts  of  an  offshore  bar.  If  the  initial  shoreline  of 
emergence  has  pronoimced  projections  or  capes,  then  the  off- 
shore bar  which  is  parallel  to  that  shoreline  will  of  necessity 
have  a  cuspate  form  imposed  upon  it.  A  study  of  the  inner 
shoreline,  back  of  the  Carolina  offshore  bars,  shows  that  the 
mainland  itself  possessed  initial  capes,  later  more  or  less  cut 
back  by  wave  action,  which  .are  perhaps  fully  competent  to 
explain  the  Carolina  cuspate  bars. 

Effect  of  Progressive  Subsidence  on  Lagoon  History.  —  If  a 
shoreline  of  emergence  bordered  by  an  offshore  bar  is  subjected 
to  a  gradual  but  continuous  subsidence,  certain  departures  from 
the  normal  history  outlined  above  may  be  noted.  Subsidence 
tendr  *o  deepen  the  water  in  front  of  the  bar,  thus  enabling 
larger  and  more  powerful  waves  to  attack  its  face.  This  must 
result  in  an  abnormally  rapid  retreat  of  the  bar,  since  a  bar 
moves  landward  just  as  fast  as  the  water  in  front  of  it  is  deep- 
ened sufficiently  to  permit  the  near  approach  of  large  waves, 
whatever  be  the  cause  of  deepening.  If  to  the  deepening  per- 
formed by  normal  wave  erosion  we  add  a  deepening  due  to 
progressive  subsidence,  certainly  the  landward  movement  of 
the  bar  will  be  appreciably  accelerated.     This  does  not  mean 


384 


DEVELOPMENT  OF  THE  SHORELINE 


s 

X 


PROGRESSIVE   SUBSIDENCE   ON   LAGOON   HISTORY      385 

that  the  lagoon  or  marsh  will  be  correspondingly  narrowed, 
since  subsidence  causes  the  inner  shoreline  to  encroach  upon 
the  land  at  the  same  time,  and  presumably  at  about  the  same 
rate  that  the  bar  moves  landward.  Both  the  bar  and  its  asso- 
ciated lagoon  or  marsh  advance  upon  the  coast  simultaneously. 
Migrating  inlets,  tidal  deltas,  and  other  shore  phenomena  de- 
velop as  before.  Sedimentation  proceeds  in  the  lagoon,  but  is 
not  so  apt  to  fill  it  as  when  the  coast  is  stationary,  since  subsi- 
dence carries  the  bottom  deposits  downward  and  continually 
renews  the  water  space  which  must  be  filled. 

When  a  marsh  has  formed  back  of  the  bar,  later  subsidence,  if 
not  too  rapid,  may  bring  about  several  peculiar  results.  In  the  first 
place,  as  the  surface  of  the  marsh  with  its  high-tide  grasses  is 
carried  downward,  new  growths  of  grass  continually  arise  upon 
the  old  in  an  effort  to  keep  the  marsh  built  up  to  the  high-tide  le\  el 
(Plate  XLV) .  The  importance  of  this  process  was  first  recognized 
by  Mudge^"  more  than  half  a  century  ago,  and  has  l-ater  been 
much  emphasized  by  C.  A.  Davis^^  The  result  is  a  deposit  of 
salt  marsh  peat,  composed  of  the  roots  and  other  remains  of 
high-tide  grasses,  whose  depth  is  an  approximate  measure  of  the 
minimum  amount  of  subsidence.  Sections  through  such  a  salt 
marsh,  instead  of  showing  high-tide  grasses  above,  remains  of 
eel-grass  and  other  low-level  grasses  immediately  below,  and 
nearly  pure  silt  or  clay  throughout  the  remaining  depth  of  the 
lagoon  deposit,  as  we  should  expect  according  to  the  Shaler 
theory  of  salt  marsh  formation,  might  show  nothing  but  remains 
of  high-tide  vegetation  from  top  to  bottom,  providing  subsidence 
had  progressed  far  enough  to  allow  the  offshore  bar  to  move 
landward  past  the  former  position  of  the  inner  shoreline,  and 
hence  beyond  the  farthest  limit  of  the  initial  lagoon  deposits. 
As  the  salt  marsh  is  progressively  built  upward  it  gradually  en- 
croaches upon  the  gently  sloping  surface  of  the  subsiding  main- 
land, overwhelming  and  burying  the  fresh-water  vegetation 
which  clothes  that  surface.  Remains  of  the  land  vegetation  may 
thus  be  preserved  as  a  layer  of  fresh-water  peat  at  the  bottom 
of  the  salt  marsh  deposit,  and  may  later  be  encountered  in  sec- 
tions cut  through  the  marsh  to  the  solid  ground  below. 

Another  consequence  of  gradual  subsidence  after  the  marsh 
has  formed  is  the  complete  disappearance  of  the  nip  along  the 
margin  of  the  mainland.     So  long  as  the  lagoon  persists,  the 


386  develop:mext  of  the  shoreline 

lagoon  waters  encroaching  upon  the  subsicHng  mainland  may  be 
sufficiently  agitated  by  winds  to  cut  a  small  cliff  at  whatever 
level  the  water  may  stand.  But  after  the  marsh  has  once  filled 
the  lagoon  area,  there  remains  no  force  capable  of  cutting  a 
straight  cliff  along  the  mainland  shore,  while  the  former  wave- 
cut  nip  is  carried  downward  under  the  marsh  by  subsidence  and 
so  lost  to  view.  Thereafter  the  marsh  surface  and  the  gently 
sloping  mainland  surface  intersect  at  a  low  angle  which  is  often 
almost  imperceptible. 

Efifect  of  Progressive  Elevation  on  Lagoon  History.  —  A 
gradual  uplift  of  the  sea-bottom,  by  decreasing  the  depth  of 
water  in  front  of  the  offshore  bar,  tends  to  cause  the  waves  to 
break  farther  and  farther  seaward.  If  the  elevation  is  so  very 
slow  that  the  normal  tendency  of  the  waves  to  deepen  the  water 
in  front  of  the  bar  by  erosion  is  not  completely  counteracted, 
the  bar  will  retreat  as  on  a  stable  coast,  but  more  slowly.  Should 
elevation  just  balance  deepening  by  wave  erosion,  we  should 
expect  the  bar  to  remain  approximately^  in  its  original  position 
while  its  crest  was  raised  higher  and  higher  out  of  the  water  and 
the  lagoon  became  dry  through  emergence.  Were  elevation 
sUghtly  more  rapid,  the  waves  would  prograde  the  bar  by 
adding  successive  ridges  to  its  face  as  they  broke  farther  and 
farther  seaward.  The  older  ridges  would  normally  have  a  higher 
average  crest  elevation,  through  uplift,  than  would  the  later 
and  hence  less  uplifted  members  of  the  series.  The  lagoon  or 
marsh  would  disappear  or  dry  up  as  the  depression  it  occupied 
was  raised  above  sealevel.  Very  rapid  elevation  might  pre- 
vent the  formation  of  well-developed  ridges  in  front  of  the 
original  bar;  or  if  the  rapid  elevation  began  before  any  bar 
had  formed,  the  bar  and  lagoon  might  not  come  into  exist- 
ence at  all  until  elevation  had  ceased  or  become  much  more 
gradual. 

Offshore  Bars  not  an  Evidence  of  Subsidence.  —  On  an 
earlier  page  we  have  referred  to  the  fact  that  certain  authors 
are  inclined  to  regard  offshore  bars  as  an  evidence  of  coastal 
subsidence.  We  are  now  in  a  position  to  return  to  this  theory, 
and  consider  it  in  the  light  of  our  discussion  of  the  normal  his- 
tory of  the  offshore  bar.  It  should  be  noted  in  the  first  place 
that  in  so  far  as  the  subsidence  theory  of  bar  formation  has 
been  elucidated  by  its  supporters,  it  would  seem  to  rest  upon 


OFFSHORE   BARS  NOT   AN   EVIDENCE  OF  SUBSIDENCE        387 

one  or  the  other  of  two  misapprehensions  regarding  the  history 
of  offshore  bars.  McGee^-  and  Ganong^''  assume  that  waves 
must  begin  to  build  up  deposits  of  sand  and  gravel  immediately 
at  the  margin  of  the  original  coast.  Offshore  bars  must  there- 
fore represent  former  coast-margin  beaches  which  have  been 
built  vertically  upward  as  the  land  subsided  and  the  receding 
shorehne  moved  inland.  The  assumption  upon  which  the  argu- 
ments of  McGee  and  Ganong  depend  for  their  validity  is,  how- 
ever, directly  opposed  to  the  conclusions  of  practically  all  stu- 
dents of  shoreline  phenomena,  to  theoretical  considerations  based 
on  the  principles  of  shoreline  development  as  outlined  above,  and 
to  observed  facts. 

It  is  not  necessarily  a  serious  objection  to  any  theory  to  say 
that  it  is  opposed  to  the  conclusions  of  former  investigators. 
Theoretical  considerations,  however,  are  directly  in  conflict  with 
the  assumption  that  offshore  bars  must  have  begun  as  ordinary 
shore  beaches  at  the  margin  of  the  mainland.  In  our  elabora- 
tion of  the  theory  of  shoreline  development  we  have  seen  that 
the  laws  of  wave  action,  according  to  which  waves  break  in  a 
depth  of  water  about  equal  to  the  wave  height,  require  a  zone 
of  breakers  some  distance  from  the  mainland  on  a  gently  sloping 
shore.  If  waves  breaking  at  the  mainland  margin  erode  the 
bottom  and  cast  up  part  of  the  debris  to  form  a  beach  ridge, 
we  should  expect  larger  waves  breaking  offshore  to  erode  the 
bottom  and  cast  up  part  of  the  debris  to  form  an  offshore  ridge 
or  bar.  Moreover,  according  to  the  theory  of  wave  action, 
subsidence,  by  deepening  the  water  in  front  of  the  wave-built 
deposit,  enables  larger  waves  to  attack  the  deposit  in  the  effort 
to  drive  it  landward.  If  waves  could  reach  the  mainland 
shore  to  build  a  beach  deposit  before  subsidence  began,  it  is 
difficult  to  see  why  more  intense  wave  action  under  the  more 
favorable  conditions  induced  by  subsidence  should  be  unable 
to  keep  the  deposit  pushed  back  to  the  same  relative  position 
as  the  shoreline  receded.  The  fact  that  the  best  development 
of  offshore  bars  is  found  where  geologically  recent  uplift  has 
brought  a  smooth,  gently  sloping  sea-bottom  within  the  zone 
of  effective  wave  action,  is  in  accord  with  what  we  should  expect 
if  the  theoretical  considerations  elaborated  on  preceding  pages 
are  correct;  whereas  the  absence  or  poor  development  of  such 
bars  on  many  coasts  known  to  have  suffered  subsidence  in  geo- 


388  DEVELOPMENT  OF  THE   SHORELINE 

logically  recent  times  is  distinctly  unfavorable  to  the  theory 
which  attributes  such  bars  to  subsidence. 

Recorded  observations  prove  that  bars  may  be  produced  by 
waves  breaking  some  distance  out  from  the  main  shoreline. 
We  have  historical  evidence  of  a  few  cases  of  this  kind  on  a 
large  scale,  such  as  the  Yarmouth  bar  on  the  east  coast  of  Eng- 
land; on  a  smaller  scale  the  process  may  be  observed  along 
the  shallow  shores  of  lakes  and  ponds.  The  writer  has  seen  a 
very  perfect  miniature  offshore  bar  formed  in  a  few  hours  by 
waves  raised  on  the  surface  of  a  small  lake  at  Lakehurst,  New 
Jersey,  during  a  fresh  breeze.  The  bar  was  a  few  inches  in 
width,  and  separated  a  shallow  lagoon  one  or  two  feet  broad 
from  the  gently  sloping  sandy  shore  which  it  paralleled  for 
some  yards. 

It  is  possible  to  read  another  meaning  into  the  words  used 
by  McGee;  and  as  this  alternate  interpretation  may  be  held 
by  others  who  regard  offshore  bars  as  proofs  of  subsidence,  we 
will  briefly  consider  it.  In  citing  offshore  bars  (which  he  calls 
"  keys  ")  as  an  evidence  of  coastal  depression,  McGee  uses  the 
phrase:  ''  the  rapidly-encroaching  sea  having  outstripped  the 
slow-moving  keys  and  left  them  far  behind^'*."  We  might  con- 
ceive this  to  mean  that  while  the  offshore  bar  was  first  formed 
by  storm  waves  some  distance  out  from  the  mainland  shore, 
and  possibly  began  to  retreat  landward  under  normal  wave 
attack,  subsidence  intervened  at  so  rapid  a  rate  that  the  inner 
shoreline  encroached  upon  the  land  faster  than  the  bar  could 
follow.  Hence,  one  might  argue,  there  is  still  a  great  breadth  of 
lagoon  or  marsh  between  the  bar  and  the  mainland,  whereas 
there  would  have  been  none  by  thiS  time  had  it  not  been  for 
subsidence. 

The  validity  of  this  argument  must  depend  upon  two  assump- 
tions: first,  that  we  know  how  long  it  normally  takes  a  bar  to 
move  from  its  initial  position  to  the  mainland  when  not  affected 
by  subsidence;  and  second,  that  the  bar  was  built  that  long 
ago.  Neither  of  these  assumptions  is  supported  by  any  evi- 
dence thus  far  brought  to  light.  We  do  not  know  the  length 
of  time  required  for  an  offshore  bar  on  a  stable  coast  to  retreat 
to  the  mainland,  nor  do  we  know  how  long  ago  the  bars  on  the 
New  Jersey  and  other  parts  of  our  coast  were  formed  We 
are  not  justified,  therefore,  in  assujning  that  the  persistence  to 


MATURE  STAGE  389 

the  present  day  of  a  lagoon  or  marsh  back  of  the  bar  is  in  any 
wise  related  to  coastal  subsidence^ 

Mature  Stage.  —  The  offshore  bar  is  a  temporary  feature, 
built  by  the  waves  because  the  initial  slope  of  the  upraised  sea- 
bottom  was  not  in  harmony  with  the  marine  forces  operating 
upon  it.  Once  the  bar  is  fully  developed,  and  the  steeper 
slope  of  its  seaward  side  is  brought  into  approximate  adjust- 
ment with  the  waves  which  break  against  it,  the  normal  retreat 
of  the  shoreline  may  begin.  Constant  loss  of  the  finer  products 
of  attrition,  which  are  swept  into  deep  water  by  current  action, 
enables  the  waves  to  drive  the  bar  slowly  landward.  Tempo- 
rary prograding  may  interrupt  the  retreat  from  time  to  time, 
as  already  explained;  but  such  interruptions  can  have  no  effect 
on  the  ultimate  history  of  the  bar.  It  is  inevitably  forced  farther 
and  farther  up  the  gentle  slope  of  the  lagoon  bottom,  or  across 
the  surface  of  the  marsh  deposits,  toward  the  initial  shoreline. 
The  advancing  waves  cut  farther  and  farther  into  the  original 
sea-bottom  in  order  to  preserve  the  same  depth  of  water  imme- 
diately in  front  of  the  retreating  bar.  A  time  must  come  when 
the  bar  has  been  forced  clear  back  upon  the  mainland,  the  lagoon 
or  marsh  has  been  wholly  destroyed,  and  the  steeper  slope  to 
deep  water  required  by  large  storm  waves  lies  just  at  the  edge 
of  the  land.  The  shoreline  of  emergence  is  then  said  to  be 
mature  (stage  H,  Fig.  119). 

Just  as  in  the  case  of  the  shoreline  of  submergence,  maturity 
of  the  shoreline  of  emergence  is  characterized  by  a  very  simple 
pattern.  Indeed,  it  is  apt  to  be  much  more  nearly  straight  for 
long  distances  than  is  the  mature  shoreline  of  submergence, 
since  it  develops  from  a  young  shoreline  which  was  itself  straight 
or  of  smiple  curvature.  The  marine  cliff  bordering  the  shore 
may  be  very  low  and  insignificant  in  early  maturity,  but  will 
increase  in  altitude  as  the  waves  cut  farther  into  the  sloping 
coastal  plain.  When  wave  attack  is  vigorous  the  cliffs  may 
themselves  be  young.  This  is  especially  apt  to  be  the  case 
during  the  early  maturity  of  the  shoreline.  A  narrow  beach 
may  intervene  between  the  base  of  the  cliff  and  the  water; 
but  owing  to  the  changing  profile  of  equilibrium  under  varying 
conditions  of  wave  attack,  the  beach  deposit  may  be  tempo- 
rarily removed  and  the  bare  rocky  surface  of  the  marine  bench 
exposed  for  a  time.     In  height  the  cliff  will  normally  be  more 


390  DEVELOPMENT  OF  THE  SHORELINE 

uniform  than  that  bordering  a  mature  shoreHne  of  submergence, 
since  it  is  carved  in  the  margin  of  a  plain  formed  by  the  com- 
paratively smooth  uplifted  sea  bottom.  The  cliff  line  will  be 
interrupted  by  the  valleys  of  those  main  streams  which  are 
sufficiently  active  to  cut  their  channels  down  to  sealevel  as 
rapidly  as  the  waves  push  the  shoreline  inland.  Smaller  and 
weaker  streams  may  descend  from  hanging  valleys  opening  well 
up  on  the  face  of  the  cliff,  the  height  of  the  valley  mouth  above 
sealevel  being  a  measure  of  the  relative  incompetency  of  the 
stream  which  occupies    it. 

It  is  hardly  probable  that  an  offshore  bar  would  retreat  at 
such  rate  in  all  its  parts  as  to  reach  the  mainland  shore  simul- 
taneously throughout  its  entire  length.  We  must  rather  expect 
that  a  stage  will  occur  when  many  parts  of  the  bar  touch  the 
mainland,  while  along  other  parts  narrow  remnants  of  the  lagoon 
still  intervene  between  bar  and  inner  shoreline.  Especially  will 
this  be  the  case  where  the  mainland  shore  was  mildly  irregular 
in  outline.  We  may  speak  of  such  a  shoreline  as  in  the  sub- 
mature  stage  of  its  development,  and  cite  the  shore  of  the  Landes 
district  of  southwestern  France  as  an  example.  The  cliffs  at 
Long  Branch  on  the  New  Jersey  coast  may  be  regarded  as 
bordering  a  mature  shoreline  of  emergence. 

One  effect  of  shoreline  retrogression  upon  the  drainage  pat- 
tern of  ji  coastal  plain  demands  a  word  in  this  connection. 
Abbe^''  has  described  the  asymmetrical  position  of  the  divides 
along  the  shores  of  Chesapeake  Bay  and  its  branches,  where 
the  water  parting  lies  nearest  to  the  shore  which  is  retreating 
most  rapidly.  This  is  due  in  part,  at  least,  to  the  fact  that 
wave  erosion  cuts  off  the  lower  ends  of  the  valleys  faster  than 
headward  stream  erosion  can  push  the  divide  back  to  a  position 
of  stable  equilibrium.  The  divide  tends  to  migrate  away  from 
that  shore  which  is  retrograding  most  rapidly;  but  its  migration 
is  sluggish  as  compared  with  the  rate  at  which  the  waves  push 
the  shoreline  toward  the  divide.  Hence  the  unsymmetrical 
position  of  the  latter. 

Old  Stage.  —  There  are  striking  differences  between  a  young 
stream  and  a  mature  stream;  but  no  such  marked  contrast 
exists  between  mature  and  old  streams.  Similarly,  while  the 
contrasts  between  young  and  mature  shorelines  are  sufficiently 
remarkable  to  call  for  much  comment,  whether  in  the  case  of 


OLD   STAGE  391 

shorelines  of  submergence  or  shorelines  of  emergence,  little  that 
is  new  can  be  said  regarding  old  shorelines  of  either  class.  The 
remarkable  lack  of  adjustment  between  shoreline  and  shore 
processes  which  characterizes  the  stage  of  youth,  is  replaced 
by  a  nearly  perfect  adjustment  in  maturity;  and  this  same 
adjustment  continues  throughout  old  age.  In  like  manner  the 
relatively  simple  shoreline  of  maturity,  bordered  on  the  one 
side  by  a  sufficient  depth  of  water  for  wave  action  close  to  the 
land,  and  on  the  other  by  marine  cliffs,  persists  into  the  latest 
stage  of  shoreline  development.  The  water  depth  immediately 
adjacent  to  the  shoreline  may  decrease  as  the  marine  bench  is 
broadened  and  the  movement  of  waves  across  it  is  retarded  in 
old  age;  the  cliff  may  weather  back  to  a  more  gentle  slope  than 
it  possessed  during  maturity,  and  hanging  valleys  may  disappear 
as  smaller'  streams  become  able  to  keep  pace  with  the  slower  re- 
treat of  the  shoreline.  But  such  changes  are  of  moderate  impor- 
tance as  compared  with  the  remarkable  transformation  which 
takes  place  between  youth  and  maturity  of  the  shoreline  cycle. 

It  must  not  be  forgotten,  however,  that  the  old  age  of  a 
shoreline  is  largely  a  matter  of  theory.  No  good  example  of  a 
shoreline  in  this  stage  of  development  is  known  to  exist  at  the 
present  time.  Young  and  mature  shorelines  are  well  known; 
and  from  them  we  may  reasonably  infer  what  some  of  the  char- 
acteristics of  old  age  must  be.  On  the  other  hand  there  are 
some  questions  concerning  which  we  must  speak  with  more 
reserve.  Thus  we  have  seen  that  as  a  land  mass  approaches 
the  condition  of  peneplanation,  the  rivers  can  bring  out  very 
little  debris,  and  waves  will  accordingly  have  less  river-brought 
material  to  deal  with.  Under  these  conditions  they  may  be 
able  to  attack  more  vigorously  the  task  of  eroding  the  coast 
and  removing  the  wave-formed  debris.  How  will  the  rate  of 
shoreline  retrogression  then  compare  with  the  earlier  rate? 
Hew  will  the  depth  of  water  near  the  shoreline,  the  slope  of  the 
marine  cliff,  and  the  condition  of  hanging  valleys  then  compare 
with  similar  features  at  an  earlier  stage  of  the  shoreline  cycle? 
We  might  discuss  such  questions  at  length  from  the  theoretical 
standpoint;  but  it  would  be  difficult  to  confirm  our  theoretical 
conclusions  by  confronting  them  with  facts  observed  in  the 
field.  Such  consideration  of  these  problems  as  appears  to  be 
profitable  has  already  been  given  in  Chapter  V. 


392  DEVELOPMENT  OF  THE  SHORELINE 

RESUME 

In  the  present  chapter  we  have  traced  the  development  of  the 
shorehne  of  emergence  from  the  initial  stage  thi'ough  its  youth 
and  maturity  to  old  age.  We  have  paused  long  enough  to  dis- 
cuss at  some  length  the  origin  of  ofTshore  bars,  and  have  con- 
cluded that  they  are  constructed  for  the  most  part  of  material 
eroded  from  the  sea-bottom  by  onshore  wave  action,  as  was  early 
stated  by  de  Beaumont;  although  the  action  of  longshore  cur- 
rents upon  which  Gilbert  relied  plays  a  significant  role  in  their 
later  history.  It  has  been  shown  that  the  theory  of  tidal  inlets 
which  would  explain  their  frequency  and  breadth  as  due  to  the 
amplitude  of  the  tidal  range,  is  in  itself  inadequate;  and  that  the 
distribution  of  inlets,  the  breadth  of  lagoons,  and  the  amount  of 
lagoon  filling  are  determined  in  part  by  the  extent  to  which 
debris  is  transported  along  the  face  of  the  offshore  bar  by  long- 
shore current  action.  A  study  of  migrating  inlets  has  developed 
the  important  conclusion  that  an  offshore  bar  broken  by  such 
inlets  may  exhibit  the  same  cross  section  after  migrating  far 
landward  over  a  salt  marsh  deposit  as  it  did  in  its  initial  stage. 
We  have  given  special  attention  to  the  effects  of  subsidence  and 
elevation  upon  offshore  bars,  lagoons,  and  marshes;  and  have 
found  that  there  is  no  support  for  the  conception  that  offshore 
bars  are  an  indication  of  coastal  subsidence. 

REFERENCES 

1.  Bryson,  John.     [On  the  beaches  along  the  southern  side  of  Long  Island.] 

Amer.  Geol.     II,  64,  1888. 

2.  Bryson,   John.     The  so-called  Sand  Dunes  of  East  Hampton,   L.   I. 

Amer.  Geol.     VIII,  188,  1891. 

3.  ScHOTT,  Arthur.     Die  Kiistenbildung  des  Xordlichen  Yukatan.     Pet. 

Geog.  Mitt.     XII,  127-130,  1866. 

4.  Agassiz,  Louis.     On  the  Relation  of  Geological  and  Zoological  Researches 

to  General  Interests,  in  the  Development  of  Coast  Features.     U.  S. 
Coast  Survey,  Rept.  for  1867,  p.  184,  1869. 

5.  Merrill,  F.  J.  H.     Barrier  Beaches  of  the  Atlantic  Coast.     Popular 

Science  Monthly.     XXXVII,  744,  1890. 

6.  McGee,  W.  J.     Encroachments  of  the  Sea.     Forum,  IX,  443,  1890. 

7.  Ganoxg,  W.  F.     Notes  on  the  Natural  History  and  Physiography  of 

New  Brunswick,  N.  B.     Nat.  Hist.  Soc,  Bull.,  No.  XXVI,  Vol.  VI, 
pt.  I,  p.  21,  1908. 

8.  GoLDTHWAiT,  J.  W.       Supposed  Evidences  of  Subsidence  of  the  Coast 

of  New  Brunswick  within  Modern  Time.     Can.  Geol.  Surv,,  Museum 
Bull.,  No.  2,  p.  7  (of  reprint),  1914. 


REFERENCES  393 

9.   Beaumont,  ;6lie,  de.      Legon?  de  Geologie  Pratique,  pp.  223-252,  Paris, 
1845. 

10.  Shaler,   N.  S.    Beaches  and  Tidal   Marshes  of  the  Atlantic  Coast 

National  Geogr.  Monogr.     I,  151-153,  1895. 

11.  Gilbert,  G.  K.     The  Topographic  Features  of  Lake  Shores.    U.  S. 

Geol.  Siu-v.,  5th  Ann.  Rep.,  p.  87,  1885. 
Gilbert,  G.  K.    Lake  Bonneville.    U.  S.  Geol.  Surv.  Mon.    I,  40,  1890. 

12.  Russell,  L  C.    Geological  History  of  Lake  Lahontan.     U.  S.  Geol. 

Surv.  Mon.     XI,  90,  1885. 
1.3.   Gilbert,  G.  K.      Lake  Bonneville.      U.  S.  Geol.  Surv.  Mon,    1,43-45, 
1890. 

14.  Ibid.,  p.  97  and  Plate  XX. 

15.  Russell,  I.  C.    Geological  History  of  Lake  Lahontan.    U.  S.  Geol. 

Surv.  Mon.     XI,  90,  1885. 

16.  Gilbert,  G.  K.    Lake  Bonneville.    U.  S.  Geol.  Surv.  Mon.     I,  40,  1890. 

17.  Davis,  W.  M.    Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  710,  Boston,  1909. 

18.  Davis,   W.   M.      Die    Erklarende  Beschreibung  der  Landformen,  pp. 

471-473,  Leipzig  and  Berhn,  1912. 

19.  Shaler,  N.  S.     Beaches  and  Tidal  Marshes  of  the  Atlantic  Coast.     Na- 

tional Geogr.  Monogr.     I,  153,  1895. 

20.  WEiDEMtJLLER,  C.  R.     Die  Schwemmlandkusten  der  Vereinigten  Staaten 

von  Nordamerika,  p.  29,  Leipzig,  1894. 

21.  Davis,  W.  M.     Physical  Geography,  p.  353,  Boston,  1898. 

22.  GuLLi\'ER,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  179,  1899. 

23.  DA\^3,  W.   M.     Die  Erklarende  Beschreibung  der  Landformen.     Fig. 

188,  Leipzig  and  Berlin,  1912. 

24.  Shaler,  N.  S.     Preliminary  Report  on  Sea-coast  Swamps  of  the  Eastern 

United  States.     U.  S.  Geol.  Surv.,  6th  Ann.  Rept.,  p.  364,  1886. 

25.  Gulliver,  F.  P.     Shoreline  Topography.     Proc.  Amer.  Acad.  Arts  and 

Sciences.     XXXIV,  183-185,  1899. 

26.  Davis,  W.  M.     Die  Erklarende  Beschreibung  der  Landformen,  p.  478, 

Leipzig  and  Berlin,  1912. 

27.  Abbe,  Cle\t;land,  Jr.     Remarks  on  the  Cuspate  Capes  of  the  CaroUna 

Cuast.     Proc.  Bost.  Soc.  Nat.  Hist.     XXVI,  496,  1895. 

28.  GuLUVER,   F.   P.     Cuspate   Forelands.     Bull.   Geol.   Soc.   Amer.    VII, 

407-410,  1896. 

29.  Davis,   W.    M.       Die   Erklarende  Beschreibung  der  Landformen,   pp. 

475-477,  Leipzig  and  Berhn,  1912. 

30.  Mudge,  B.  F.     Salt  Marsh  Formation  of  Lynn.      Essex  Institute  Proc. 

II,  117-119,  1862. 

31.  Davis,  Chas.  A.     Salt  Marsh  Formation  near  Boston  and  its  Geological 

Significance.     Economic  Geology-,  V,  623-639,  1910. 
DavXS,  Chas.  A.     Some  Evidences  of  Recent  Subsidence  on  the  New 

England  Coast.     Science  New  Ser.,  XXXII,  63,  1910. 
Davis,  Chas.  A.     Salt  Marshes,  a  Study  in  Correlation.     Assoc.  Am. 

Geographers,  Annals.     I,  139-143,  1911. 


394  DEVELOPMENT  OF  THE  SHORELINE 

32    McGee,  W.  J.     Encroachments  of  the  Sea.     Forum,  IX,  443,  1890 
Sa'   Ganong,  W.  F.    Notes  on  the  Natural  History  and  Physiography  of 
New  Brunswick,  N.  B.     Nat.  Hist.  Soc,  BuU.  No.  XXVI,  Vol.  VI, 

34    McGee.^W  J.     Encroachments  of  the  Sea.     Forum,  IX,  443,  1890 
35".   Abbe,  Cleveland.     A  General  Report  on  the  Physiography  of  Mary- 
land.    Maryland  Weather  Service.     I,  104,  1899. 


CHAPTER  VIII 

DEVELOPMENT   OF   THE   SHORELINE  (Continued) 

C.   NEUTRAL   AND    COMPOUND   SHORELINES 

Neutral  Shorelines.  —  It  would  not  be  appropriate  in  the 
present  volume  to  discuss  at  length  the  developmental  history  of 
all  the  different  types  of  neutral  shorelines.  The  general  princi- 
ples outlined  under  the  preceding  discussion  of  shorelines  of  sub- 
mergence and  shorelines  of  emergence  present  a  foundation  upon 
which  the  student  may  base  a  treatment  of  any  neutral  shore- 
line, making  such  minor  modification  of  treatment  as  the  special 
peculiarities  of  the  particular  type  may  warrant.  We  may  note 
in  passing,  however,  that  alluvial  plain  and  outwash  plain  shore- 
lines, like  the  shoreline  of  the  coastal  plain,  have  a  simple  pat- 
tern in  the  initial  as  well  as  in  later  stages;  but  that  unlike  the 
latter  type,  they  need  not  pass  through  an  offshore  bar  stage 
because  of  their  steeper  seaward  slope  from  the  water  margin. 
Lobate  delta  shorelines  pass  through  a  submature  stage  in  which 
an  arcuate  pattern  is  given  to  the  outer  shoreline  by  the  build- 
ing of  bars  connecting  the  seaward  extremities  of  the  lobes. 
Portions  of  the  Rhone  and  Nile  deltas  appear  to  possess  shore- 
lines representing  this  stage  of  development.  A  true  arcuate 
delta  shoreline  may  characterize  the  mature  stage  of  a  lobate 
delta  shoreline,  if  wave  erosion  cuts  back  the  lobes  beyond  the 
heads  of  the  inter-lobe  bays  (Fig.  120).  An  appreciation  of  the 
variety  of  delta  types  responsible  for  some  of  the  variations  in 
delta  shorelines  may  be  gained  from  an  inspection  of  Credner's 
well-known  essay  on  "  Die  Deltas^  ". 

A  valuable  discussion  of  delta  formation  is  given  by  Barrell  in  an 
essay  on  "  Criteria  for  the  Recognition  of  Ancient  Delta  Deposits." 
The  "delta  cycle"  is  thus  briefly  summarized  by  this  author: 
"  In  the  stage  of  youth  before  the  drainage  system  has  become 
well  developed  the  detritus  delivered  at  the  river  mouth  is  some- 
what smaller  in  amount  but  coarser  in  texture.  The  subaqueous 
wave-cut  profile  is  also  undeveloped,  the  bottom  still  inheriting 

395 


396 


DEVELOPMENT  OF  THE  SHORELINE 


its  original  slope.  If  this  initial  slope  is  gentler  than  the  sub- 
aqueous profile  of  equilibrium  the  waves  have  at  first  less  power 
of  erosion  at  the  coast  line.  If  the  initial  slope  is  steeper  they 
will  possess  an  initially  greater  power.  Assuming,  however, 
that  the  river  is  dominant  over  the  sea,  the  delta  is  rapidly 


Fig.  120.  —  Diagram  showing  how  wave  erosion  of  a  lobate  delta  may  trans- 
form it  into  an  arcuate  delta  (broken  line). 

built  outward,  and  on  accoimt  of  the  coarse  waste,  the  steeper 
river  grades,  and  shallow  bottom  near  shore,  the  initial  propor- 
tion of  the  subaerial  topset  beds  is  relatively  high.  During 
maturity  the  quantity  of  waste  is  larger,  as  all  parts  of  the 
drainage  system  now  supply  sediment,  but  as  the  river  is  graded 
and  its  gradient  is  also  flattened  the  waste  is  finer  in  texture. 
The  delta  is  extended  outward  and  the  greater  deposit  is  on  the 
outer  portions.  It  grows  inland  also  for  a  time,  but  owing  to 
the  flattening  grade  the  beds  in  this  direction  show  decreasing 
thickness.  The  maximum  rate  of  outward  growth  is  reached 
early  because  of  the  increasing  surface  area,  which  requires  a 
greater  volume  of  sediment  to  give  a  unit  thickness,  and  the 
increasing  depth  of  the  water,  which  involves  a  continually 
deeper  fill.  Furthermore,  the  increasing  shoreline  and  greater 
exposure  to  the  waves  increase  the  power  of  the  latter  to  carry 
away  the  waste,  which  with  the  progress  of  the  cycle  becomes 
finer  in  texture  and  more  readily  removed  by  the  sea.  But 
although  the  rate  of  advance  falls  off,  the  outward  growth  will 
continue  during  the  progress  of  maturity  in  the  cycle  of  erosion 
and  deposition.  In  old  age,  however,  on  account  of  the  ever- 
slackening  supply  of  waste  and  the  larger  portion  carried  in 


NEUTRAL  SHORELIXES 


397 


suspension  and  solution,  the  sea  at  last  gains  the  mastery  and 
begins  to  plane  inland  across  the  low-lying  and  unconsolidated 
materials  projecting  into  the  sea.  Rapid  headway  is  finally 
made  against  the  weakened  river;  the  territory  conquered  by 
the  river  in  its  youth  is  reclaimed  and  the  sea  at  last  will  beat 
once  more  against  the  margin  of  the  old  land- ". 

The  development  of  fault  shorelines  has  been  ably  discussed 
by  Cotton^,   who  presents  a  detailed   analysis  of   the   features 


Fig.  121.  —  Fault  shoreline  bordering  a  scarp  which  dies  out  toward  the 
right.  The  fault  traversed  a  region  of  strong  relief.  (Modified  after 
Cotton.) 

characterizing  successive  stages  in  the  life  history  of  such  shore- 
lines. As  he  is  careful  to  point  out,  the  initial  character  of  the 
fault  shoreline  will  vary  widely  according  as  the  fault  traverses 
a  maturely  dissected  land  mass  of  strong  relief  (Fig.  121),  or  an 
undissected  coastal  plain  of  no  appreciable  relief  (Fig.  122). 
In  either  case  streams  betrunked  by  faulting  will  cascade  into 
the  sea  from  the  mouths  of  hanging  valleys.  Thus  the  initial 
stage  of  fault  shorelines  resembles  the  mature  stage  of  shore- 
lines of  submergence  in  cases  where  the  relatively  simple  cliff- 
line  of  the  latter  type  is  marked  by  hanging  valleys  due  to 


398 


DEVELOPMENT  OF  THE  SHORELINE 


rapid  wave  attack.  The  character  of  the  seaward  slope,  how- 
ever, is  very  different  in  the  two  cases.  Where  the  landward 
block  bordering  a  fault  shoreline  has  itself  been  partially  de- 
pressed, thereby  bringing  the  main  valley  floors  at  the  fault 
scarp  down  to  sealevel,  there  will  be  no  large  hanging  valleys. 


Fig.  122.  —  Similar  to  Fig.  121,  except  that  the  fault  traversed  a  little- 
dissected  plain  of  faint  relief.     (Modified  after  Cotton.) 

If  the  landward  block  has  been  depressed  sufficiently  to  permit 
the  sea  to  enter  and  submerge  the  main  valleys,  we  have  an 
initial  compound  shoreline  (Fig.  124),  the  treatment  of  wliich 
type  is  reserved  for  a  later  section.  Seaward  from  the  fault 
scarp  the  sea  floor  will  have  the  contours  of  the  pre-faulting  land 
surface,  whether  that  be  an  uTegular  sm-face  (Fig.  121)  or  a 
smooth  plain  (Fig.  122).  The  seaward  slope  in  the  immediate 
vicinity  of  the  shoreline  will  normally  be  very  steep,  as  it  is  the 
slope  of  the  fault  scarp  itself. 

Wave  attack  on  the  fault  scarp  will  not  proceed  very  rapidly 
at  fu-st,  both  because  steep  walls  rising  out  of  deep  water 
tend  to  reflect  waves,  and  because  the  waves  are  unarmed  with 
rock  fragments  with  which  to  make  then-  attack  more  effective. 
The  face  of  the  cliff  will  weather  back  to  a  more  moderate  slope, 
and  the  weathering  products  will  accumulate  at  the  cliff  base  as 
a  subaqueous  talus.  Streams  emptying  from  hanging  valleys  will 
rapidly  entrench  themselves,  cutting  young  gorges  in  the  more 
mature  valleys   of  the  initial  land  surface,   thus  producing  a 


NEUTRAL  SHORELINES 


399 


typical  two-cycle  topography  without  necessarily  implying  any 
change  in  the  level  of  the  land  area  in  question  or  of  the  adjacent 
water  surface.  The  erosion  products  brought  out  by  the  streams 
will  accumulate  as  subaqueous  talus  cones  which  may  later 
take  the  form  of  ordinary  deltas.  With  the  shallowing  of  the 
bottom  near  shore  by  accumulations  of  debris  derived  from 
fault  face  and  stream  valleys,  wave  reflection  is  less  perfect 
and  wave  attack  more  vigorous,  particularly  since  supplies  of 


Fig.  123.  —  Successive  stages  in  the  retrogression  of  a  fault  shoreline 
bordering  rocks  of  varying  resistance. 

rock  fragments  are  now  accessible  to  the  waves.  The  retreat 
of  the  shoreline  takes  place  more  rapidly  for  a  time.  Later, 
when  the  marine  bench  and  shoreface  terrace  have  attained  a 
considerable  width,  the  vigor  of  the  waves  traversing  them  is 
somewhat  reduced  for  reasons  explained  on  earlier  pages;  and 
the  marine  cliff,  steepened  while  wave  attack  was  increasing  in 
vigor,  has  opportunity  to  weather  back  to  a  more  gentle  slope. 
The  fault  shoreline  has  now  reached  maturity,  and  henceforth 
develops  in  the  same  manner  as  other  mature  shorelines.  As 
noted   under  shorelines  of  submergence,   a  shoreline  bordering 


400  DEVELOPMENT  OF  THE  SHORELINE 

weak  rock  areas  will  retreat  more  rapidly  than  one  bordering 
regions  of  resistant  rock.  From  this  it  follows  that  an  initially- 
straight  fault  shoreline  may  acquire  a  pattern  of  simple  curves 
in  maturity,  the  re-entrant  curves  being  systematically  related 
to  weak  rock  areas  (Fig.  123). 

Compound  Shorelines.  —  Thus  far  we  have  considered  the 
developmental  stages  of  shorelines  of  submergence,  shorelines  of 
emergence,  and  neutral  shorelines.  It  remains  only  to  point  out 
very  briefly  any  special  features  characteristic  of  different  stages 
of  compound  shorelines,  or  those  shorelines  which  exhibit  promi- 
nently features  normally  characteristic  of  at  least  two  of  the 
foregoing  classes. 

In  its  young  stage  a  compound  shoreline  combining  features 
of  both  submergence  and  emergence  will  be  characterized  by 
an  offshore  bar  which  determines  a  straight  outer  shoreline,  and 
drowned  valleys  which  give  an  irregular  inner  shoreline.  The 
bar  may  be  broken  by  tidal  inlets  and  possess  all  the  other  fea- 
tures of  such  a  bar  on  a  shoreline  of  emergence.  Similarly,  the 
lagoon  may  in  course  of  time  become  filled  with  sediment  or 
marsh  deposits.  Whether  or  not  such  filling  occurs,  the  various 
types  of  spits,  forelands,  and  bars  which  are  so  marked  a  feature 
of  young  and  submature  shorelines  of  submergence,  are  largely 
lacking  along  the  irregular  inner  part  of  the  compound  shore- 
line, for  the  reason  that  the  offshore  bar  protects  the  inner  shore 
from  the  effective  wave  action  which  is,  as  we  have  already 
seen,  largely  responsible  for  these  shore  forms. 

Such  a  compound  shoreline  may  be  called  submature  when 
the  offshore  bar  has  been  driven  against  the  headlands  of  the 
inner  shore.  The  bays  between  the  headlands  will  then  appear 
to  be  closed  by  bay  bars;  and  cases  may  occur  in  which  a  sub- 
mature  compound  shoreline  could  not  be  distinguished  from  a 
submature  shoreline  of  submergence.  In  the  latter  type  of  shore- 
line, however,  the  bars  closing  the  different  bays  have  devel- 
oped more  or  less  independently;  and  it  is  doubtful  whether 
they  will  ordinarily  show  that  relatively  straight  alignment  char- 
acteristic of  the  different  parts  of  a  single  offshore  bar  which  has 
been  driven  against  the  headlands  of  the  irregular  inner  part  of 
a  compound  shoreline.  Maturity  is  reached  when  outer  and 
inner  shoreline  have  coalesced  in  one  shoreline  back  of  the  heads 
of  the  initial  embayments.     From  this  time  on  the  features  of 


CONTRAPOSED  SHORELINES 


401 


the  compound  shoreline  do  not  differ  from  those  of  the  shoreline 
of  emergence. 

A  compound  shoreline  combining  features  of  a  fault  shore- 
line with  those  of  a  shoreline  of  submergence  (Fig.  124)  passes 
through  a  first  stage  in  which  the  outer  or  fault  shoreline  por- 
tion develops  like  any  normal  fault  shoreline,  while  the  drowned 
valley  portions  of  the  partially  submerged  block  have  the  same 
history  as  the  more  deeply  indented  portions  of  normal  shore- 


TiG.  124.  —  Compound  slioreline,  combining  essential  features  of  a  shore- 
line of  submergence  and  a  fault  shoi-eline. 


line  of  submergence.  Maturity  is  reached  when  wave  erosion 
has  pushed  the  initial  fault  scarp,  later  become  a  normal  marine 
cliff,  back  of  the  bay  heads,  and  a  simple  shoreline  bordered  by 
a  continuous  marine  cliff  is  developed.  Further  stages  of  de- 
velopment show  no  peculiar  features. 

Contraposed  Shorelines.  —  If  a  coastal  region  of  hard  rocks  is 
separated  from  the  sea  by  a  belt  of  overlapping  softer  deposits, 
as  where  a  rugged  oldland  is  overlapped  by  a  narrow  coastal 
plain,  the  shoreline  which  is  first  developed  upon  the  softer  beds 
will  later  be  retrograded  until  it  comes  against  the  hard  rocks. 
Such  a  shorehne  has  well  been  called  "  contraposed  "  by  C.  H. 
Clapp^,  and  in  origin  it  is  analogous  to  a  "  superposed  "  river 
which  has  been  let  down  from  a  soft  rock  cover  upon  under- 
lying ridges  of  harder  material.  A  shoreline  which  has  reached 
maturity  in  the  softer  beds  may  in  its  contraposed  position  lose 


402 


DEVELOPMENT  OF  THE  SHORELINE 


im<  W^ii  ir.^  if  W.:  i(ftC\«aFJ-.'-  :3Bi>   .V  '.'  V..-  -  , 


^  s^ 


K 


Q 


REFERENCES 


403 


its  mature  characteristics  and  acquire  those  of  youth  (Fig.  125). 
It  may  even  change  from  a  typical  shorehne  of  emergence  to  one 
having   the   characteristics   of   submergence,    if   the   older   and 


Fig.  125.  —  Stages  in  the  formation  of  a  contraposed  shoreline.     Early 
stage  shown  by  upper  figure.     (Modified  after  Clapp.) 

harder  rocks  possessed  a  very  rugged  surface  and  the  soft  rock 
mantle  consisted  of  unconsolidated  material  easily  removed. 
Parts  of  the  New  England  shoreline  belong  to  the  contraposed 
type  (Plate  XL VI). 

REFERENCES 

1.  Credner,  G.  R.     Die  Deltas.     Pet.  Geog.  Mitt.,  Erganzung.sband  XII, 

No.  56,  1-74,  1878. 

2.  Barrell,  Joseph.     Criteria  for  the  Recognition  of  Ancient  Delta  De- 

posits.    Bull.  Amer.  Geol.  Soc.     XXIII,  397,  1912. 

3.  Cotton,  C.  A.     Fault  Coasts  in  New  Zealand.     The  Geographical  Re- 

view.    I,  20-47,  1916. 

4.  Clapp,  C.  H.     Contraposed  Shorehnes.     Jour,  of  Geol.     XXI,  537,  1913, 


CHAPTER  IX 

SHORE   RIDGES   AND   THEIR   SIGNIFICANCE 

Advance  Summary.  —  Many  beaches,  bars,  tombolos  and  fore- 
lands are  characterized  by  a  succession  of  narrow  ridges  built  by 
the  waves,  and  sometimes  later  modified  by  the  winds.  These 
"lines  of  growth"  of  shore  forms  have  much  significance  for  the 
engineer  who  would  learn  something  of  current  action  and  direc- 
tion of  debris  movement  at  a  given  locality  in  the  recent  past, 
and  for  the  geological  or  geographical  student  who  would  trace 
the  development  of  shore  forms  and  ascertain  what  light  they 
may  throw  upon  the  important  question  of  past  changes  in  the 
relative  levels  of  land  and  sea.  It  is  the  purpose  of  the  present 
chapter  to  discuss  the  origin  of  beach  ridges  and  dune  ridges;  to 
inquire  into  the  rate  at  which  they  have  been  formed,  with  the 
hope  of  acquiring  data  useful  in  estimating  the  ages  of  those 
shore  forms  which  possess  them;  and,  finally,  to  analyze  in  a 
critical  manner  the  conditions  under  which  beach  ridges  may  be 
used  to  determine  whether  coasts  have  recently  experienced  appre- 
ciable changes  of  level. 

Origin  of  Beach  Ridges.  —  Beach  ridges  have  long  been 
recognized  as  representing  successive  positions  of  an  advancing 
shoreline,  and  are  known  to  the  English  as  "fulls";  while  the 
depressions  between  them  are  known  as  "  swales,"  "  slashes," 
or  "  furrows."  When  a  beach  ridge  is  covered  by  dune  sands 
we  have  a  "  dune  ridge  ";  the  swales  between  dune  ridges  have 
been  called  "  dune  valleys  "  (Diinentaler)  by  the  Germans.  -Un- 
usually good  examples  of  beach  ridges  or  dune  ridges  are  found 
on  Orford  Ness  on  the  east  coast  of  England  as  described  by 
Redman^;  on  the  Dungeness  foreland  of  the  southeast  coast, 
described  by  Drew-,  Redman^,  and  Gulliver^;  .on  the  Darss  fore- 
land of  the  Baltic  coast  of  Germany  described  by  Otto^  in  a 
paper  on  "  Der  Darss  and  Zingst  ";  at  Swinemiinde  on  the  same 
coast  where  a  most  remarkable  series  of  dune  ridges  has  been 
described  by  Keilhack^  in  a  most  interesting  essay  entitled  "  Die 

404 


ORIGIN  OF  BEACH   RIDGES  405 

Verlandung  der  Swinepf orte " ;  and  on  Cape  Canaveral  off  the 
east  coast  of  Florida.  Beach  or  dune  ridges  are  usually  found  in 
a  greater  or  less  degree  of  perfection  on  any  prograded  shoreline, 
and  they  are  of  so  much  importance,  not  onl}^  in  showing  the 
successive  stages  of  development  of  the  forms  which  possess  them, 
but  also,  as  will  presently  appear,  in  showing  whether  a  coast 
has  remained  stable  or  experienced  changes  of  level  during  their 
formation,  that  it  is  pertinent  to  inquire  somewhat  fully  into 
their  origin  and  significance. 

According  to  Gilbert^  a  wave-built  terrace  or  beach  plain  is 
usually  produced  whenever  a  shoreline  maintains  its  course 
while  the  longshore  current  diverges.  "  The  surface  of  the 
wave-built  terrace,  considered  as  a  whole,  is  level,  but  in  detail 
it  is  uneven,  consisting  of  parallel  ridges,  usually  curved.  Each 
of  these  is  referable  to  some  exceptional  storm,  the  waves  of 
which  threw  the  shore  drift  to  an  unusual  height^ ".  The 
forward  building  of  the  shore  occurs  because  the  diverging 
current  assumes  a  greater  cross  section  and  a  diminished  veloc- 
ity; and  with  diminished  velocity  an  accumulation  of  the  trans- 
ported debris  must  take  place.  "  This  accumulation  occurs, 
not  at  the  end  of  the  beach,  but  on  its  face,  carrying  its  entire 
profile  lakeward  and  producing  by  the  expansion  of  its  crest  a 
tract  of  new-made  land." 

Davis^  explains  the  prograding  of  an  offshore  bar  by  suppos- 
ing that  waves  breaking  on  a  shallowing  sea  floor  cast  up  the 
bottom  material  into  an  initial  bar  or  ridge;  later,  larger  storm 
waves  break  a  little  farther  out  in  deeper  water,  and  from  the 
ne\vly  eroded  bottom  material  construct  another  bar  on  the 
face  of  the  earlier  one.  "  A  preliminary  offshore  bar  is  built 
up  by  the  storm  waves  .  .  .  ;  and  afterwards,  at  times  of 
exceptional  storms,  successive  additions  may  be  made  on  its 
outer  side^"."  According  to  this  theory,  also,  the  accumulation 
occurs  on  the  face  and  not  on  the  end  of  the  earlier  deposit;  but 
the  material  is  supposed  to  be  derived  from  the  sea-bottom  and 
not  from  the  longshore  currents  upon  which  Gilbert  relied. 

When  a  recurved  spit  develops  into  a  compound  spit  or  fore- 
land by  the  addition  of  successive  spits  or  embankments  to  its 
seaward  side,  there  is  produced  a  beach  plain  characterized  by 
sub-parallel  ridges  separated  by  belts  of  lower  land  or  strips  of 
water.     In  this  case,  however,  the  accumulation  may  take  place 


406 


SHORE   RIDGES  AND   THEIR   SIGNIFICANCE 


> 

< 


ORIGIN  OF  BEACH   RIDGES  407 

not  simultaneously  along  the  entire  face  of  the  earlier  deposit, 
but  by  extension  of  the  ends  of  the  successively  formed  embank- 
ments; longshore  currents  furnish  the  principal  supply  of  mate- 
rial; and  the  individual  ridges  are  evidently  not  to  be  correlated 
with  a  corresponding  number  of  great  storms.  Davis^i  appears 
to  regard  the  beach  ridges  of  the  Provincelands  as  having  been 
produced  in  the  manner  here  indicated,  although  his  admirable 
essay  on  "  The  Outline  of  Cape  Cod"  does  not  explicitly  state 
that  the  successive  embankments  all  grew  longitudinally  from 
their  point  of  tangency  with  the  mainland  cliff. 

It  is  highly  probable  that  ridged  beach  plains  have  been 
produced  in  all  three  of  the  ways  mentioned  above.  Where 
one  part  of  a  shore  is  being  cut  back  and  straightened  by  the 
waves,  a  longshore  current  may  have  its  course  so  modified  as 
to  depart  from  an  adjacent  section  of  the  shore  which  it  pre- 
viously followed.  If  the  withdrawal  is  gradual  enough,  the 
portion  of  the  shore  affected  may  continuously  be  prograded  by 
deposits  laid  down  in  the  manner  described  by  Gilbert.  Where 
the  withdrawal  is  more  rapid,  successive  separate  embank- 
ments may  build  a  compound  spit  or  foreland  bar.  Interme- 
diate forms  between  these  two  types  must  exist.  Sandy  Hook 
in  its  earlier  development  appears  to  have  consisted  of  several 
embankments  built  independently  from  southeast  to  northwest, 
one  after  the  other;  but  in  later  years  it  seems  possible  that  its 
whole  seaward  face  has  advanced  eastward  at  times  by  practi- 
cally simultaneous  deposition  along  its  length.  There  is  likewise 
good  reason  to  believe  that  some  offshore  bars  have  been  slightly 
prograded  by  the  building  of  one  or  more  embankments  in  the 
deeper  water  outside  of  the  original  bar,  after  the  manner  sug- 
gested by  Davis.  In  the  opinion  of  the  present  writer,  however, 
the  processes  described  above  are  not  the  only  ones,  nor  perhaps 
the  most  important  ones,  by  which  ridged  beach  plains  are  pro- 
duced; nor  should  the  beach  ridges  in  any  case  be  regarded  as 
the  product  of  individual  great  storms,  as  has  been  so  commonly 
assumed. 

It  has  already  been  shown  in  connection  with  the  discus- 
sion of  beach  profiles  of  equilibrium,  that  a  shoreline  must  be 
prograded  wherever  longshore  currents  of  any  type  bring  to 
it  more  debris  than  the  waves  there  operating  can  remove. 
Deposition  of  excess  debris  shallows  the  olTshore  bottom,  favor- 


408  SHORE  RIDGES  AND   THEIR   SIGXIFICAXCE 

ing  the  forniation  of  waves  of  translation,  which  in  turn  drive 
the  bottom  debris  on  shore  until  prograding  of  the  shoreline 
and  deepening  of  the  bottom  produces  a  profile  which  is  in 
equilibrium  with  the  forces  there  at  work.  If  the  supply  of 
debris  by  longshore  currents  is  kept  up  indefinitely,  the  shore 
may  be  extensively  prograded  before  equilibrium  is  established. 
It  should  be  noted  that  in  the  case  here  considered  the  long- 
shore currents  are  forced  to  move  seaward  because  the  shore  is 
prograded,  whereas  in  the  case  mentioned  by  Gilbert  the  shore 
was  prograded  because  the  cm'rents  moved  seaward.  Waves 
are  the  active  agents  in  causing  the  prograding,  and  derive 
much  of  their  material  from  the  offshore  bottom,  as  in  the  case 
of  offshore  bars  mentioned  by  Davis.  But  unlike  the  case  con- 
sidered by  him,  longshore  currents  are  primarily  responsible  for 
a  continuous  supply  of  material  which  as  continuously  shallows 
the  offshore  bottom;  and  the  prograding  of  the  shore  is  not  to 
be  correlated  with  the  initial  bottom  slope  nor  with  storm 
waves  of  different  sizes.  The  shoreline  advances  seaward 
th]"oughout  a  considerable  portion  of  its  extent  simultaneously, 
and  does  not  grow  by  the  longitudinal  extension  of  each  ridge, 
as  in  the  case  of  compound  spits. 

It  is  immaterial  what  particular  type  or  types  of  currents 
bring  an  excess  of  debris  to  the  prograding  area.  Beach  drift- 
ing along  both  the  shore  and  shoreface  zones  is  an  exceedingly 
important  and  commonly  neglected  source  of  supply.  Where 
beach  drifting  is  from  opposite  directions  toward  a  common 
point,  as  not  infrequently  happens  in  bays  and  lakes,  there  will 
be  an  accumulation  of  material  at  the  meeting  point,  where 
weaker  or  conflicting  wave  currents  are  unable  to  dispose  of  it. 
Beach  drifting  in  but  one  direction  along  a  shoreline  which  sud- 
denly changes  its  trend,  will  cause  an  excessive  deposit  just  be- 
yond the  angle  in  case  the  shore  bends  backward,  because  wave 
action  upon  the  more  protected  shore  around  the  bend  is  not 
sufficiently  vigorous  to  remove  all  the  debris  deposited  there 
Material  drifted  along  bayside  beaches  toward  the  bay  heads, 
shallows  the  latter  areas  and  permits  the  small  waves  operat- 
ing there  actively  to  prograde  the  shoreline. 

Offshore  bars  are  characteristic  of  shorelines  of  emergence, 
and  are  described  on  an  earlier  page.  But  it  will  not  be  in- 
appropriate to  consider  in  this  connection  the  origin   of   pro- 


ORIGIN  OF  BEACH  RIDGES  409 

graded  bars  showing  beach  ridges.  Where  an  offshore  bar  has 
been  formed  with  a  profile  of  equihbriuni  nicely  adjusted  to  the 
marine  forces  along  its  entire  length,  it  is  evident  that  any 
disturbance  of  conditions  at  one  point  along  its  sea  front  may 
lead  to  retrograding  or  prograding  at  another.  A  succession 
of  storms  causing  unusual  erosion  in  one  locality  may  permit 
beach  drifting  or  other  longshore  movements  to  carry  an  exces- 
sive amount  of  debris  to  another  part  of  the  bar,  disturbing  the 
equilibrium  there  and  causing  prograding.  The  opening  and 
closing  of  inlets,  by  affecting  longshore  transportation,  may 
indirectly  cause  retrograding  or  prograding  on  adjacent  parts 
of  the  shore.  Additions  to  the  face  of  an  offshore  bar  do  not 
necessarily  imply,  therefore,  that  larger  storm  waves  have  been 
breaking  on  the  deeper  parts  of  an  initial  sloping  sea-bottom; 
neither  does  retrograding  indicate  that  the  bar  previously  ad- 
vanced to  the  zone  where  the  largest  storm  waves  broke  on 
the  initial  bottom,  and  that  it  has  now  entered  a  new  stage  of 
its  development  characterized  by  progressive  retreat.-  On  the 
coatrary,  both  retrograding  and  prograding  must  frequently 
be  interpreted  as  horizontal  oscillations  of  the  shoreline  conse- 
quent upon  disturbances  of  the  shore  profile  of  equilibrium 
which  may  be  very  temporary  in  some  cases,  but  endure  for  a 
considerable  time  in  others.  As  will  be  shown  in  later  chapters, 
parts  of  the  Atlantic  shoreline  have  repeatedly  been  retro- 
graded and  prograded.  It  follows  from  these  considerations 
that  the  retrograding  or  prograding  of  a  shore  does  not  form  a 
satisfactory  basis  for  discriminating  between  stages  of  shoreline 
development,  as  has  been  sometimes  assumed. 

It  may  happen  that  an  initial  shallow  on  a  shoreline  of  sub- 
mergence will  for  a  long  time  occasion  the  formation  of  waves 
of  translation,  which  will  in  turn  sweep  upon  the  shore  all  debris 
deposited  ovar  the  shallow.  A  cuspate  foreland  may  thus  ad- 
vance over  the  shallow  and  finally  conceal  it,  with  the  result 
that  the  shore  will  exhibit  a  foreland  unrelated  to  any  visible 
shore  irregularity  or  any  known  currents.  A  river  may  deposit 
so  much  sediment  opposite  its  mouth  as  to  shallow  the  sea- 
bottom,  whereupon  the  waves  will  re-establish  the  shore  profile 
of  equilibrium  by  eroding  the  bottom  and  prograding  the  shore- 
line, the  latter  action  producing  a  cuspate  foreland  (or  cuspate 
delta)  showing  parallel  beach  ridges  (Fig.  126). 


TT* 


Ta^UxurLentxf  River 


Fig.  126.  —  Cuspate  delta  of  the  Tagliamento  River,  Italy,  showing  parallel 

beach  ridges. 
Page  410 


ORIGIN   OF  BEACH   RIDGES  411 

There  is  a  widespread  belief  that  the  beach  ridges,  which 
often  characterize  the  surface  of  forelands,  bars,  tombolos,  and 
other  prograded  shore  forms,  represent  the  work  of  individual 
great  storms.  Men  whose  opinions  must  always  carry  great 
weight  have  either  explicitly  or  by  imphcation  supported  this 
view.  Gilbert^^  [^  yg^.y  q\q^j.  [j^  j^^g  statement:  "  Each  of  these 
(ridges)  is  referable  to  some  exceptional  storm,  the  waves 
of  which  threw  the  shore  drift  to  an  unusual  height."  Davis^^ 
expresses  much  the  same  opinion  concerning  beach  ridges  on 
oifshore  bars,  but  adds  that  further  study  and  observation  are 
required  to  demonstrate  the  validity  of  certain  points  in  his 
explanation  of  bar  formation.  Other  authors  have  expressed 
somewhat  different  views.  In  discussing  a  paper  by  Redman'^ 
on  the  shore  deposits  along  the  south  coast  of  England,  B.  S. 
Howlett^'^  states  that  every  beach  ridge  represents  "  the  accu- 
mulation of  shingle  resulting  from  some  stormy  tide,"  while 
Sir  William  Cubitt^*'  ['  apprehended  that  these  '  fulls  '  coin- 
cided with,  or  at  least  were  influenced  to  some  extent,  by  the 
lunar  cycles."  Cornish^^  would  recognize  "  neap  tide  fulls  " 
and  "  spring  tide  fulls."  He  apparently  considers  that  these 
tidal  ridges  may  be  amalgamated  into  a  '-summer  full  "  and  a 
"  winter  full,"  and  that  these  larger  fulls  may  in  their  turn 
sometimes  coalesce.  Unlike  most  observers,  Wheeler^^  believes 
that  the  ridges  were  built  up  during  calm  weather.  Solger^^ 
advances  the  theory  that  in  the  case  of  dune  ridges,  which  as 
we  shall  see  later  are  essentially  beach  ridges  capped  by  sand 
dunes,  each  ridge  was  formed  during  a  dry  climatic  period, 
when  the  sand  of  a  prograding  shore  was  blown  back  to  the 
line  of  ridge  formation;  v/hile  the  intervening  swales  represented 
wet  periods  during  which  vegetation  advanced  rapidly  over  the 
newly  gained  land  and  prevented  the  sand  from  being  blown 
into  dunes.  Three  dune  ridges  are  supposed  by  Solger  to  be 
formed  each  century,  each  of  which  corresponds  to  the  dry 
phase  of  the  well-known  35-year  climatic  period  of  Bruckner. 
Keilhack^o  estimates  that  at  Swinepforte  one  ridge  has  formed 
in  every  35  years  on  the  average,  and  he  follows  Solger  in  corre- 
lating their  formation  with  the  35-year  Bruckner  cycle. 

There  are  several  reasons  for  doubting  the  possibility  of  cor- 
relating individual  beach  ridges  with  a  corresponding  number 
of  exceptional  storms  which  cast  up  the  shore  drift  to  an  unusual 


il2  SHORE  RIDGES  AND   THEIR   SIGNIFICANCE  ' 

height.  In  the  first  place,  it  is  difficult  to  imagine  the  supply 
of  shore  debris  and  other  shore  conditions  so  adjusted  that  each 
exceptional  storm  would  find  enough  material  available  with 
which  to  construct  a  high  ridge,  yet  too  much  to  permit  the 
ridge  to  be  driven  back  into  coalescence  with  an  earlier  one 
formed  by  the  last  preceding  exceptional  storm.  On  the  con- 
trary, we  should  rather  expect  that  one  exceptional  storm  might 
do  no  more  than  raise  a  submarine  bar  in  front  of  the  shore; 
a  second  great  storm  from  a  slightly  different  direction  might 
wipe  the  bar  out  of  existence;  the  bar  might  reform  during  a 
third  storm  of  equal  violence;  moderate  waves  in  calmer  weather 
might  then  raise  the  surface  of  the  bar  into  a  ridge  a  number 
of  feet  above  sealevel;  the  next  great  storm  might  produce  a 
new  bar  in  front  of  the  one  just  formed;  and  so  on.  In  this 
imaginary  case  there  occurred  four  exceptional  storms,  but  there 
are  only  two  beach  ridges;  and  one  of  these  was  not  raised  above 
the  sea  by  any  of  the  storms.  Observation  will  show  that  many 
beach  ridges  when  followed  along  their  crests  subdivide  into 
two  or  more  ridges.  Manifestly,  if  the  separate  ridges  be  re- 
garded as  the  work  of  several  exceptional  storms,  the  compound 
ridge  cannot  properly  be  regarded  as  the  work  of  one  storm. 
The  number  of  ridges  formed  in  a  given  time  do  not  correspond 
with  the  expectalile  number  of  great  storms  within  that  period. 
Thus  the  121  ridges  of  the  Darss  foreland  in  Germany  have 
been  built  in  a  period  estimated  to  be  from  3000  to  6000  years 
which  would  mean  an  average  of  only  one  great  storm  in 
every  25  or  50  years.  If  the  time  required  for  the  develop- 
ment of  Nantasket  beach  has  been  correctly  estimated  by 
Johnson  and  Reed-^  one  would  have  to  suppose  that  only  one 
great  storm  in  several  centuries  has  been  recorded  by  the  beach 
ridges  in  the  southern  half  of  that  district. 

It  is  clearly  impossible  to  suppose  that  every  great  storm 
builds  a  beach  ridge,  for  observation  abundantly  proves  the 
contrary.  Indeed,  I  know  of  no  case  in  which  a  typical  com- 
plete beach  ridge  of  large  size  has  been  wholly  produced  by  one 
storm,  although  I  do  not  regard  this  as  impossible.  On  the 
other  hand,  a  large  part,  if  not  all,  of  a  beach  ridge  is  often 
swept  away  during  a  single  exceptional  storm.  We  cannot 
suppose  that  every  beach  ridge  represents  the  work  of  one 
exceptional  storm,  since,  as  has  been  shown,  such  a  ridge  often 


ORIGIN  OF   BEACH   RIDGES  413 

represents  the  combination  of  several  ridges  elsewhere  distinct, 
I  do  not  believe  that  one  should  even  regard  a  given  beach 
ridge  as  necessarily  the  product  of  several  exceptional  storms; 
for  while  unusually  high  beach  ridges  must  have  been  subjected 
to  the  influence  of  waves  of  sufficient  magnitude  to  cast  debris 
to  their  crests,  the  majority  of  ridges  could  have  reached  their 
present  height  through  the  influence  of  ordinary  storm  waves, 
and  many  of  them  perhaps  by  very  moderate  wave  action  at 
high  tide.  It  is  even  possible  to  suppose  that  on  a  given  beach 
plain  none  of  the  exceptional  storms  of  the  past  are  recorded 
by  any  of  the  ridge  crests,  but  only  the  more  prolonged  activities 
of  less  violent  wave  action. 

The  height  of  a  beach  ridge  depends  in  part  upon  the  size  of 
waves,  but  in  part  also  upon  other  factors,  among  which  may 
be  mentioned  rapidity  of  supply  of  material,  and  the  relation 
of  the  new  ridge  to  pre-existing  ridges.  If  longshore  currents 
supply  debris  with  great  rapidity,  the  shoreline  may  be  pro- 
graded  so  fast  that  a  given  beach  ridge  has  little  opportunity 
to  grow  to  a  great  height  before  the  shoreface  zone  is  shallowed 
and  a  new  ridge  begins  to  form  in  front  of  it.  A  number  of 
ridges  of  moderate  height  might  thus  be  formed  in  the  intervals 
between  exceptional  storms.  Continued  shallowing  of  the  off- 
shore zone  due  to  rapid  deposition  would  also  tend  to  change 
the  largest  storm  waves  into  smaller  waves  of  translation  before 
they  reached  the  line  of  ridge  building,  with  the  result  that 
even  great  storm  waves  might  not  build  high  ridges.  Less 
rapid  supply  of  shore  debris  would  favor  the  building  of  higher 
ridges  in  several  ways;  waves  could  cast  material  upon  the 
ridge  nearly  as  fast  as  it  was  supplied,  enabhng  a  ridge  to  grow 
to  its  full  height  before  sufficient  change  occurred  in  the  shore 
profile  to  require  the  initiation  of  a  new  ridge  farther  seaward; 
great  storm  waves  would  have  a  better  opportunity  to  reach 
the  shoreline,  and  the  longer  life  of  a  ridge  at  the  shoreline 
would  increase  the  chances  of  such  waves  assisting  in  its  con- 
struction; and  while  slower  debris  supply  would  increase  the 
danger  of  ridge  removal  by  storm  waves,  it  would  also  increase 
the  chances  that  wave  attack  might  drive  the  shoreward  ridge 
back  upon  the  one  behind  it,  thus  forming  a  compound  ridge  of 
greater  height.  It  can  hardly  be  doubted  that  many  of  the 
prominent  beach  ridges  of  prograded  shores  represent  the  accu- 


414  SHORE  RIDGES  AND  THEIR   SIGNIFICANCE 

mulations  of  many  subordinate  beach  ridges  successively  formed 
in  front  of  a  main  shore  ridge  and  later  driven  back  upon  it. 

The  future  of  any  given  beach  ridge  is  very  uncertain,  because 
of  the  variable  nature  of  the  marine  forces  operating  upon  a 
prograding  shore.  It  may  have  its  further  growth  arrested  by 
the  development  of  another  ridge  in  front  of  it;  it  may  be  com- 
pletely washed  away  by  the  next  storm;  it  may  grow  until  it 
acquires  large  size  and  permanence  of  position;  or  it  may  be 
driven  back  to  coalesce  with  one  or  more  earlier  ridges.  A 
ridged  beach  plain  is  thus  a  very  imperfect  record  of  a  complex 
history :  only  a  fraction  of  the  ridges  once  formed  are  preserved ; 
the  records  of  many  storms  are  forever  lost;  some  of  the  re- 
maining ridges  may  record  one  great  storm,  others  certainly 
represent  the  work  of  many  different  wave  attacks  upon  the 
same  line,  while  still  others  are  composed  of  two  or  more  formerly 
independent  ridges  forced  into  coalescence.  One  may  admit 
that  beach  ridges  can  be  materially  affected  by  great  storms,  by 
spring  and  neap  tides,  by  summer  and  winter  storms,  and  pos- 
sibly even  by  a  35-year  climatic  cycle;  but  he  must  still  recog- 
nize the  impracticability  of  correlating  a  given  series  of  ridges 
with  a  given  succession  of  any  of  these  phenomena. 

Rate  of  Beach  Ridge  Formation.  —  The  student  of  shorelines 
often  desires  to  secure  an  approximate  idea  of  the  length  of 
time  w^hich  has  elapsed  since  the  sea  worked  upon  a  certain 
part  of  the  coast,  and  a  succession  of  beach  ridges  sometimes 
affords  the  best  available  data.  It  is  occasionally  possible  to 
determine  the  time  occupied  in  building  a  certain  number  of 
the  latest  ridges,  and  if  the  rate  were  uniform  throughout  the 
growth  of  the  entire  beach  plain  the  problem  would  be  a  simple 
one.  From  what  has  been  said,  however,  it  is  evidently  far 
from  safe  to  assume  that  the  older  ridges  were  formed  at  the 
same  rate  as  those  of  later  date.  The  history  of  a  beach  plain 
is  too  complex,  and  its  record  preserved  in  too  incomplete  a 
manner,  to  enable  one  to  sa}^  how  few  or  how  many  ridges  have 
been  eliminated  by  erosion  or  coalescence.  Furthermore,  the 
rate  of  debris  supplj^  must  vary  with  time,  and  the  increasing 
depth  of  water  encountered  as  the  plain  builds  forward  into 
the  sea  must  affect  its  rate  of  growth.  There  are,  nevertheless, 
certain  general  principles  which  may  guide  one  in  endeavoring 
to  reach  a  reasonable  conclusion  as  to  the  approximate  time 


RATE  OF  BEACH   RIDGE   FORMATION 


415 


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416  SHORE   RIDGES  AND   THEIR  SIGNIFICANCE 

represented  by  a  given  series  of  ridges;    these  may  be  stated 
categorically,  with  such  comments  as  seem  necessary. 

1.  Short  ridges  normally  require  less  time  than  longer  ones. 
Thus  a  series  of  short  ridges  representing  successive  recurved 
points  at  the  end  of  a  spit  may  succeed  each  other  with  rapidity, 
since  all  the  debris  carried  along  the  shore  is  concentrated  at 
the  narrow  end  of  the  spit.  The  same  amount  of  debris  em- 
ployed in  prograding  a  long  stretch  of  the  shoreline  would 
build  very  few  long  ridges  in  the  same  length  of  time.  Rock- 
away  Beach,  near  the  entrance  to  New  York  Harbor,  is  a  good 
example  of  a  compound  recurved  spit  which  is  growing  west- 
ward at  a  fairly  rapid  rate  by  the  addition  of  successive  recurved 
ridges  of  small  height  at  its  distal  point.  As  will  appear  from 
Figure  127,  reproduced  from  survey  charts  and  reduced  to  a 
common  scale  by  the  Metropolitan  Sewerage  Commission  of 
New  York  City,  the  westernmost  ridge  of  the  1889  chart  was 
duninished  in  area  and  two  new  ridges  were  added  before  the 
survey  of  1905.  Seven  years  later  three  additional  ridges  had 
been  formed.  In  other  words,  five  ridges  were  formed  in  twenty- 
three  years,  the  average  rate  of  growth  varjdng  from  one  ridge 
in  eight  years  to  one  ridge  in  a  little  more  than  two  years.  The 
actual  increase  in  the  length  of  the  spit  during  the  whole  period 
was  nearly  one  mile,  or  an  average  annual  advance  of  over 
200  feet. 

2.  For  a  given  exposure,  low  and  narrow  ridges  imply  a 
smaller  lapse  of  time  than  an  equal  number  of  high,  broad 
ridges.  This  depends  upon  the  fact  already  explained  that 
rapid  supply  of  debris  tends  to  cause  a  rapid  prograding  of  the 
shoreline,  with  opportunity  for  low  and  narrow  ridges  only  to 
form. 

3.  One  series  of  parallel  ridges  abruptly  truncated  by  another 
series  trending  in  a  different  direction  (Plate  XLVIII),  does  not 
necessarily  imply  a  longer  lapse  of  time  than  would  a  single  par- 
allel series  containing  the  same  total  number  of  ridges.  This  will 
be  apparent  from  Figure  128.  Let  us  imagine  that  a  projecting 
headland,  with  the  shoreline  0,  0,  is  cut  back  on  its  north  side  to 
the  new  shoreline  1,  and  that  the  eroded  debris  is  deposited  on 
the  east  to  form  the  beach  ridge  1'.  Later  erosion  cuts  the  shore 
farther  back  to  2,  thereby  removing  the  extreme  northern  end  of 
the  beach  ridge  1',  while  deposition  of  the  eroded  debris  forms  beach 


Fig.  127.  —  Successive  stages  in  the  development  of  Rockaway  sand 
spit,  Long  Island. 

Page  417 


418 


SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 


ridge  2'.  This  process  is  repeated,  until  erosion  drives  back  the 
shore  to  5,  thereby  truncating  the  northern  ends  of  beach  ridges 
numbers  1'  to  4'  inclusive,  and  deposition  forms  beach  ridge  5', 
Owing  to  a  change  in  the  balance  of  shore  processes,  possibly 
consequent  upon  a  change  outside  of  the  area  shown  in  the 
figure,  deposition  replaces  erosion  on  the  north,  and  the  beach 


i'V  2'3'4'5'6'7'8 


Fig.  128.  —  Diagram  of  cliffed  headland  and  associated  beach  ridge  plain, 
showing  that  one  series  of  ridges  truncating  another  does  not  necessarily 
imply  a  longer  lapse  of  time  than  an  equal  number  of  parallel  ridges. 


ridge  6,  6'  is  formed  all  around  the  headland,  being  followed  by 
ridges  7,  7'  and  8,  8'.  We  now  have  on  the  north  one  series  of 
parallel  ridges  which  abruptly  truncates  another  series;  yet  no 
greater  time  is  here  represented  than  that  represented  by  the 
continuously  parallel  series  1'  to  8'  measured  toward  the  east. 
It  is  not  safe  to  assume,  as  has  sometimes  been  done,  that  where 
one  series  of  ridges  truncates  another,  allowance  must  be  made 
for  a  large  time  interval  at  the  break.  Discordance  of  ridge 
direction  may  or  may  not  imply  a  greater  lapse  of  time  than 
accordance. 

4.  Dune  ridges,  or  parallel  ridges  of  dune  sand  corresponding 
in  all  respects  with  beach  ridges,  except  as  regards  details  of  sur- 
face form,  are  to  be  regarded  as  resting  upon  true  beach  ridges, 
and  may  be  used  as  readily  as  the  latter  in  interpreting  shoreline 


RATE  OF   BEACH   RIDGE  FORMATION  419 

changes.  The  regularity  of  crestlines  and  parallel  arrangement 
of  the  dune  ridges  in  such  regions  as  the  Darss  foreland  and 
Cape  Canaveral  (Fig.  129)  leave  no  doubt  that  they  are  lines  of 
shore  dunes  which  have  formed  on  the  successive  beach  ridges 
when  each  ridge  was  next  to  the  sea.  Trenches  cut  through 
dune  ridges  have  revealed  the  presence  of  beach  sands  or  gravels 
below.  That  the  dune  ridges  have  not  moved  from  ^their  initial 
position  is  evident,  for  had  they  done  so  their  crests  would  have 
become  very  irregular  and  would  necessarily  have  lost  their 
beautiful  parallelism.  On  Cape  Cod,  where  the  dunes  of  the 
Provincelands  have  migrated  under  the  influence  of  the  winds, 
their  former  parallelism  is  lost  and  the  position  of  the  beach 
ridges  is  scarcely  determinable.  The  idea,  sometimes  advanced, 
that  the  ridges  are  merely  Hues  of  shore  dunes  which  have 
rolled  inland  from  a  stationary  shoreline  like  waves  of  the  sea, 
will  not  commend  itself  to  those  familiar  with  the  phenomena 
of  dune  migration.  Since  the  dunes  must  have  formed  in  place 
on  beach  ridges  at  the  shore,  there  must  have  been  time  enough 
in  each  case  for  a  beach  ridge  to  be  formed  by  the  waves;  prob- 
ably also  for  enough  vegetation  to  gain  a  foothold  on  the  ridge 
to  arrest  windblown  sand  coming  from  the  beach  and  so  prevent 
its  being  carried  over  into  the  swale  or  slash  back  of  the  ridge; 
and,  finally,  for  the  dune  sand  to  accumulate  in  sufficient  quan- 
tity materially  to  augment  the  height  of  the  ridge.  Long, 
broad,  and  high  dune  ridges,  like  those  of  the  Darss  or  Canav- 
eral, must  have  required  many  years  for  their  construction. 

5.  While  a  large  number  of  beach  ridges  indicates  the  lapse 
of  a  long  time  interval  since  the  first  one  was  formed,  the  con- 
verse is  not  true.  On  a  graded  shoreline,  where  neither  pro- 
grading  nor  retrograding  is  occurring,  a  single  beach  ridge  may 
represent  the  slow  accumulation  of  many  centuries.  Not  in- 
frequently two  or  three  beach  ridges  on  one  part  of  a  shore  rep- 
resent the  same  time  interval  as  does  a  large  series  of  beach 
ridges  on  a  closely  adjacent  part  of  the  shore.  It  is  not  permis- 
sible, therefore,  to  assume  a  short  time  interval  for  the  building 
of  narrow  beach  plains  containing  but  few  ridges. 

Dungeness  Cuspate  Foreland.  —  It  may  not  be  without  interest 
to  review  some  of  the  data  available  as  to  actual  or  estimated 
rates  of  beach  ridge  and  dune  ridge  formation.  The  Dungeness 
of  southeaster^  England  is  a  prominent  cuspate  foreland  project- 


CAPE 
CANAVERAL 


Fig.  129.  —  Ridges  of  the  Cape  Canaveral  cuspate  foreland.    For  the  most 
"•  part  they  are  dune  ridges,  but  beach  ridges  little  altered  by  wind  action 

occur    near  the  Light. 
Page  420 


RATE  OF  BEACH  RIDGE  FORMATION 


421 


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422  SHORE  RIDGES  AND   THEIR  SIGNIFICANCE 

ing  from  a  curved  reentrant  of  the  shore  and  measuring  about 
15  miles  along  either  side,  its  seaward  portion  consisting  of  a 
splendid  series  of  shingle  beach  ridges.  Between  certain  groups 
of  the  ridges  are  broad  belts  of  marsh,  while  the  base  of  the  fore- 
land consists  almost  wholly  of  marshland  formed  by  the  silting 
up  of  an  extensive  bay,  which  formerly  occupied  the  interior  of 
an  initial  compound  cuspate  bar.  As  a  rule  the  shingle  ridges 
are  covered  by  ver}^  little  vegetation,  although  some  of  the  older 
ones  are  grassed  over;  while  belts  of  grass  and  broom  occupy 
many  of  the  swales,  thereby  emphasizing  to  the  eye  the  ridged 
character  of  the  surface.  (Plate  XLIX.)  In  general  appearance 
the  ridges  and  swales  closely  resemble  the  well  known  "  Wallen" 
and  "  Rinnen  "  of  the  island  of  Riigen  (Plate  L)  described  and 
figured  by  Braun".  So  far  as  I  could  judge  without  careful 
measurements,  the  ridge  crests  of  the  Dungeness  are  prevailingly 
of  moderate  height,  possibly  rising  3  to  6  feet  above  intervening 
swales  and  8  to  12  feet  above  high  tide,  where  typically  devel- 
oped. Occasional  sandy  ridges  are  encountered,  but  well  rounded 
flint  shingle  is  the  only  material  found  in  most  of  the  ridges. 

As  shown  by  the  map  (Fig.  130)  the  older  ridges  have  clearly 
been  truncated  by  wave  erosion  on  the  south  side  of  the  fore- 
land or  "  ness,"  and  the  erosion  products  built  into  additional 
ridges  at  the  point  and  along  the  east  side.  Two  miles  west  of 
the  point  the  ridges  show  a  complex  arrangement  over  a  broad 
area,  but  along  a  line  drawn  from  Lj^dd  to  the  point  of  the  ness 
the  succession  of  ridges  is  fairly  regular.  During  the  reign  of 
Elizabeth,  the  distance  from  Lydd  church  to  the  extremity  of 
the  point  was  three  miles,  according  to  Redman^^.  In  1860,  as 
shown  by  sheet  No.  4  of  the  Geological  Survey  of  Great  Britain, 
this  same  distance  was  nearly  four  miles,  indicating  that  the 
point  advanced  seaward  about  one  mile  in  a  little  less  than  three 
centuries,  which  is  equivalent  to  an  annual  advance  of  a  little 
over  6  yards.  Redman^*  studied  the  rate  of  advance  as  indi- 
cated by  various  lines  of  evidence  accessible  to  him  in  1852  and 
concluded  "  that  the  average  annual  increase,  during  two  cen- 
turies has  at  least  amounted  to  nearly  6  yards."  Drew^^  found 
that  from  1794  to  1860  the  annual  advance  was  about  5^  yards. 

There  are  about  25  beach  ridges  showai  on  Drew's  map  (Sheet 
4,  Geological  Survey  of  Great  Britain),  as  crossing  the  last  mile 
of  the  distance  from  Lydd  to  the  point  of  the  ness.     Although 


RATE   OF  BEACH  RIDGE   FORMATION 


423 


424  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 

Drew  states  that  south  and  southeast  of  Rye  the  ridges  are  more 
numerous  than  could  be  shown  upon  the  map''',  in  discussing 
the  changes  near  the  point  of  the  Dungeness  he  says  that  he 
"  inserted  all  the  '  fulls '  or  shingle  ridges  on  the  previously 
featureless  Ordnance  map^^."  Gulliver-^  counted  twenty-three 
''successive  shorelines"  on  the  east  side  of  the  ness  between 
Lydd  and  the  sea,  and  as  the  ridges  there  cover  a  breadth 
of  about  a  mile,  and  are  shown  by  the  Ordnance  map  to  be 
between  20  and  25  in  number,  it  would  seem  fair  to  assume 
that  near  the  point  of  the  Dungeness  one  ridge  was  built  on  an 
average  every  11  or  12  years.  It  should  be  noted  that  some  of 
the  ridges,  especially  those  closest  to  the  point,  are  short,  and 
that  they  are  formed  of  material  easily  and  rapidly  secured  from 
the  south  side  of  the  ness  which  has  long  been  suffering  active 
erosion;  both  of  which  facts  would  lead  us  to  expect  an  unusu- 
ally rapid  development  of  ridges  near  the  point.  That  this  has 
been  the  case  is  suggested  by  Redman's  observation  in  1852 
that  the  point  had  advanced  with  unusual  rapidity  during  the 
two  years  previous  to  his  study-^  although  the  period  is  too 
short  to  be  ver}^  significant.  One  of  the  coast  guards  stationed 
on  the  south  shore  of  the  ness  informed  me  that  the  sea  had  re- 
moved their  lookout  house  and  cut  that  part  of  the  coast  back 
50  feet  within  recent  years,  while  the  east  side  of  the  ness  was 
advancing  about  20  yards  annually.  This  is  in  apparent  dis- 
agreement with  Gulliver's  statement  in  1897  that  recent  obser- 
vation indicated  an  annual  advance  of  but  1|  yards^";  but  both 
figures  may  be  correct  for  limited  periods. 

The  second  edition  of  Le win's  "  Invasion  of  Britain  by  Julius 
Csesar^^  "  contains  an  interesting  map,  reproduced  by  Burrows^^ 
in  his  volume  on  Cinque  Ports,  which  shows  the  location  of 
marshy  lands  on  the  Dungeness  reclaimed  previous  to  the  14th 
century.  From  this  map  it  appears  that  the  Denge  Marsh, 
east  of  Lydd,  was  dyked  about  774  A.D.  Since  this  marsh 
could  hardly  have  come  into  existence  until  at  least  one  beach 
ridge  had  formed  to  the  east  of  the  present  position  of  the  marsh 
to  shut  out  the  vigorous  waves  incident  to  such  an  exposed 
locality,  it  would  appear  that  the  ridges  east  of  Lydd,  already 
stated  to  be  23  in  number,  have  been  formed  in  the  interval 
between  a  date  previous  to  774  and  the  present  time.  This  would 
mean  an  average  of  about  50  years  for  the  construction  of  each 


RATE  OF  BEACH  RIDGE  FORMATION 


425 


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426  SHORE   RIDGES   AND  THEIR  SIGNIFICANCE 

beach  ridge.  Drew^^  considers  that  the  region  east  of  Lydd 
was  open  sea  up  to  the  tenth  or  eleventh  centuries,  and  while 
his  arguments  are  not  wholly  conclusive  on  this  point,  it  may 
be  noted  that  on  the  basis  of  his  interpretation  each  ridge  re- 
quired not  more  than  35  to  40  years  for  its  construction.  That 
the  older  ridges  southwest  of  Lydd  are  of  considerable  an- 
tiquity is  indicated  by  the  weathered  character  of  their  com- 
ponent pebbles^^. 

An  attempt  has  been  made  to  show  that  the  Dungeness  did 
not  exist  at  all  in  the  time  of  Julius  Csesar,  and  Appach^^ 
gives  a  map  of  the  supposed  condition  of  this  part  of  the  English 
coast  in  the  year  55  B.C.  upon  which  the  foreland  does  not 
appear.  Should  this  contention  be  valid,  then  the  ridges  of  the 
Dungeness,  numbering  in  1860  at  least  135  according  to  a  map 
which  probably  does  not  show  the  full  number,  must  all  have 
formed  within  an  interval  of  little  more  than  1900  years;  or  at 
an  average  rate  of  one  ridge  in  14  years.  There  are  ample 
grounds  for  rejecting  Appach's  conclusions,  however.  He  did 
not  properly  understand  the  processes  by  which  the  Dungeness 
was  formed,  and  his  methods  of  reasoning  are  unconvincing. 
The  fact  that  certain  towns  formerly  seaports  are  now  far 
inland,  upon  which  he  bases  some  of  his  arguments  in  favor  of 
the  recent  construction  of  the  foreland,  is  readily  explained 
by  Lewin's  map  which  shows  navigable  bays  back  of  the  beach 
ridges  of  Dungeness  point.  The  towns  were  located  upon 
bays,  which  have  since  silted  up  and  been  converted  into 
dyked  marshes.  Roman  remains  are  found  extensively  over 
Romney  Marsh  which  occupies  the  northern  half  of  the  foreland, 
proving  that  a  large  part  of  the  Dungeness  was  completed  and 
under  cultivation  in  Roman  times^^.  Robertson^'^  has  likewise 
demonstrated  that  much  of  the  Dungeness  existed  at  this  an- 
cient period.  This  means  that  the  construction  of  the  beach 
ridges  of  the  entire  foreland  occupied  an  unknown  length  of 
time,  certainly  greater  than  2000  years,  and  probably  very 
much  greater. 

The  available  data  accordingly  indicates  that  the  rate  of  beach 
ridge  formation  on  the  Dungeness  foreland  has  varied  greatly 
at  different  times,  the  average  rate  over  a  number  of  years 
rising  as  high  as  one  ridge  every  11  or  12  years  at  certain  times 
and  places,  and  dropping  at  least  as  low  as  one  ridge  in  40  or  50 


RATE  OF   BEACH   RIDGE   FORMATION 


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.428  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 

years  elsewhere.  One  must  fully  recognize,  however,  that  even 
at  a  given  place  and  period  the  building  of  ridges  is  neither 
uniform  in  rate  nor  necessarily  continuously '  forward.  In  a 
series  of  ridges  formed  at  the  average  rate  of  one  every  12  years 
a  certain  ridge  may  have  required  half  a  century  or  more  for 
its  completion,  several  other  ridges  may  all  have  been  built 
within  a  decade,  while  still  others  may  have  been  built  and  later 
destroyed  by  a  temporary  erosion,  thereby  lowering  the  aver- 
age rate  of  ridge  formation  for  the  series  as  a  whole.  For  this 
reason,  rates  of  beach  ridge  formation  based  on  data  covering 
very  short  periods  are  not  of  much  value. 

Taking  all  the  facts  into  consideration  I  am  inclined  to  be- 
heve  that  an  average  rate  of  one  ridge  constructed  every  20  to 
40  years  is  probably  a  reasonable  figure  for  the  Dungeness  as  a 
whole. 

Darss  Cuspate  Foreland.  —  With  the  exception  of  Cape  Canav- 
eral, the  finest  example  of  a  cuspate  foreland  composed  largely 
of  dune  ridges  which  it  has  been  my /good  fortune  to  see,  is  the 
Darss  foreland  northwest  of  Stralsund  on  the  Baltic  coast  of 
Germany.  Several  former  islands  are  here  tied  to  each  other 
and  to  the  mainland  by  a  complex  tombolo,  which  has  been  pro- 
graded  in  front  of  the  principal  island  (the  Alt  Darss)  to  form  a 
triangular  cuspate  foreland  (the  Neu  Darss)  measuring  from  7 
to  10  kilometers  (4  to  6  miles)  on  each  side.  Northeasterly  and 
easterly  moving  beach  drifting,  possibly  aided  by  other  currents, 
transported  debris  which  wave  action  built  into  a  series  of  beach 
ridges,  the  axis  of  each  ridge  trending  first  northeast  and  then 
eastward.  After  each  beach  ridge  was  constructed  dry  sands 
from  the  shore  were  blown  upon  its  crest  by  the  winds  until  it 
rose  into  a  dune  ridge  from  one  to  several  meters  in  height.  As 
the  foreland  grew  northward  into  the  Baltic,  erosion  along  its 
western  side  removed  large  portions  of  the  ridges  in  that  direc- 
tion while  redeposition  of  the  eroded  material  on  its  northern 
side  accelerated  the  northward  advance.  So  much  of  the  western 
ends  of  the  ridges  has  been  lost  by  erosion  that  the  east  trend- 
ing portions  alone  remain  to  make  up  most  of  the  resulting 
truncated  cuspate  foreland  (Fig.  131). 

Unlike  the  barren  shingle  ridges  of  the  Dungeness,  the  dune 
ridges  of  the  Darss  are  well  forested  (Plate  LI),  and  the  "  Darss- 
erwald  "   is  now   protected   as   a   hunting   preserve  for  one  of 


RATE  OF   BEACH  RIDGE  FORMATION 


429 


the  German  princes.  The  crests  of  the  ridges  rise  15  to  25 
feet  above  the  adjacent  swales  in  places,  and  occasional  ridges 
and  a  number  of  individual  dunes  reach  a  greater  altitude.  Most 
of  the  ridges  do  not  exceed  a  height  of  10  feet  above  the  deepest 
parts  of  the  swales,  and  perhaps  the  greater  number  fall  short 


Fig.  131.  —  Dune  ridges  of  the  Dar.ss  cuspate  foreland,  Germany. 


of  6  feet.  Some  of  the  swales  are  deep  enough  to  contain  long 
narrow  ponds,  others  are  marshy,  while  still  others  differ  from 
the  pine  covered  ridges  in  having  fewer  trees  and  a  grassy  bot- 
tom. Ordinarily  the  ridges  are  from  75  to  150  feet  apart,  but 
this  distance  varies  greatly  in  different  parts  of  the  Darss, 
swales  between  500  and  1000  feet  in  breadth  being  known.  The 
roads  through  the  forest  are  sandy,  and  where  they  are  cut 
through  the  higher  ridges  one  occasionally  sees  a  good 'exposure 
of  cross  bedded  dune  sands,  the  surface  layers  being  bleached 
by  weathering  in   the  ridges  earliest  formed;    but  ferns  and 


430  SHORE  RIDGES  AND   THEIR  SIGNIFICANCE 

other  vegetation  usually  carpet  the  forest  floor  and  conceal 
the  sand,  making  the  region  one  of  great  beauty.  There  is 
little  in  the  forest  covering  to  remind  one  of  the  scrub  palmetto 
and  occasional  palms  of  Cape  Canaveral;  but  in  spite  of  the 
contrast  in  vegetation,  the  forms  of  the  dune  ridges  and  swales, 
the  variation  in  ridge  height  and  spacing,  and  the  greater 
weathering  of  the  sands  in  the  older  dunes,  constantly  reminded 
me  of  identical  features  observed  in  the  Canaveral  ridges  only  a 
few  months  previously. 

The  Darss  has  been  briefly  described  by  Braun^^  and  at 
great  length  by  Otto^^.  The  excellent  essay  of  the  latter  author, 
entitled  "  Der  Darss  und  Zingst:  Ein  Beitrag  zur  Entwick- 
lungsgeschichte  der  Vorpommerschen  Kuste,"  is  based  upon  a 
comparative  study  of  ancient  and  modern  maps  and  detailed 
field  investigations;  and  the  author  discusses  at  length  the 
preglacial  conditions  of  the  region  involved,  the  effects  of  gla- 
ciation  and  of  post-glacial  changes  of  level  upon  the  coastal 
topography,  and  finally  the  more  recent  morphological  changes 
of  the  coast  including  the  development  of  the  dune  ridges. 
Unfortunately  it  is  sometimes  impossible  to  follow  all  of  this 
author's  arguments,  because  he  commits  the  too  common  error 
of  locating  important  features  and  describing  essential  meas- 
urements in  terms  of  unimportant  local  roads,  property  bound- 
aries, etc.,  the  names  of  which  do  not  appear  on  any  maps  in 
his  report  nor  on  any  other  maps  available  to  the  ordinary 
reader. 

Otto's  description  of  the  dune  ridges*"  is  open  to  the  criticism 
just  mentioned;  but  I  understand  from  the  text  that  there  are 
121  dune  ridges  distinguishable  in  passing  from  south  to  north 
along  the  western  side  of  the  Darss,  only  a  part  of  vyhich  number 
are  indicated  on  the  German  topographic  map  of  the  area. 
Historical  evidence  proves  that  the  coast  has  advanced  1300 
feet  (400  meters)  in  200  years.  Using  this  figure  as  a  basis 
for  calculation,  and  making  some  allowance  for  the  fact  that 
the  younger  dune  ridges  were  probably  built  more  rapidly  than 
the  older  ones,  Otto  concludes  that  3000  years  is  the  shortest 
possible  time  in  which  the  121  ridges  could  have  been  con- 
structed*^  This  would  mean  an  average  of  at  least  25  years  for 
the  construction  of  each  ridge.  Otto  allows  1000  years  addi- 
tional for  the  formation  and  subsequent  destruction  of  some 


RATE  OF  BEACH  RIDGE   FORMATION  431 

older  ridges  at  the  immediate  base  of  the  foreland,  and  thus 
arrives  at  the  conclusion  that  the  submergence  which  initiated 
the  period  of  dune  ridge  formation  (the  "  Litorinasenkung  ") 
occurred  at  least  4000  years  ago,  or  as  early  as  2000  B.C.  If 
Keilhack'*^  is  more  nearly  correct  in  his  opinion  that  this  period 
of  submergence  occurred  7000  years  ago,  as  seems  probable  to 
the  writer  for  reasons  which  will  subsequently  appear,  then  the 
construction  of  the  121  ridges  of  the  Darss  occupied  something 
like  twice  the  minimum  period  assigned  by  Otto,  and  the  aver- 
age time  for  building  each  ridge  would  be  nearly  50  years. 

Swinemunde  Tombolo.  —  The  magnificent  series  of  dune  ridges, 
which  make  up  the  complex  tombolo  *  connecting  the  islands  of 
Usedom  and  Wollin  some  distance  east  of  the  Darss  has  been 
mentioned  in  many  German  works  dealing  with  sand  dunes,  and 
is  described  at  considerable  length  in  Solger's  "  Dunenbuch-*^" 
A  strait  some  eight  miles  or  more  in  width  formerly  separated  the 
two  islands.  Northerly  winds  blowing  across  a  broad  stretch  of 
open  water  would  drive  upon  the  converging  shores  of  the  islands 
vigorous  waves,  which  would  in  turn  cause  active  beach  drifting 
southeastward  along  the  northeast  shore  of  Usedom  and  south- 
westward  along  the  northwest  shore  of  Wollin.  Two  spits  began 
to  advance  into  the  strait,  the  western  or  Swinemunde  spit  trer  d- 
ing  nearly  due  south  along  the  east  shore  of  Usedom,  while  the 
eastern  or  Misdroy  spit  extended  itself  in  a  more  westerly 
direction  across  the  strait,  being  strongly  recurved  southward 
at  the  point.  The  Swinemunde  spit  was  then  extensively  pro- 
graded  to  form  a  beach  plain  by  the  addition  of  some  80  dune 
ridges  to  its  seaward  side,  the  Misdroy  spit  meantime  advancing 
by  gaining  150  successive  recurved  points  at  its  western  end 
while  its  seaward  side  was  being  retrograded.  When  the  strait 
was  nearly  closed,  erosion  truncated  the  northern  end  of  the 
Swinemunde  beach  plain,  cut  back  the  mainland  shore  of  Use- 
dom some  distance,  and  possibly  continued  the  previous  trun- 
cation of  the  Misdroy  recurved  points.  There  followed  a  pro- 
grading  of  both  the  Swinemunde  and  Misdroy  areas,  by  which 

*  The  fact  that  a  narrow  stream  passes  between  the  islands  by  a  channel 
eroded  across  some  of  the  dune  ridges  does  not  alter  the  fact  that  the  islands 
are  essentially  connected  by  a  beach  plain  which  is  continuous  just  below 
water  level,  even  if  interrupted  by  the  stream  at  the  surface;  hence  I  have 
called  the  combined  complex  spits  a  tombolo. 


432  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 


RATE  OF  BEACH  RIDGE   FORMATION  433 

additional  series  of  30  and  40  dune  ridges  respectively  were 
added  to  the  northern  sides  of  the  almost  united  spits.  Erosion 
slightly  truncated  these  later  ridges,  and  a  third  and  last  series 
was  then  added,  bringing  the  completed  tombolo  to  its  present 
form.  A  narrow  stream,  the  Swine,  which  flows  alternately 
northward  and  southward  is  all  that  remains  of  the  former 
strait,  and  it  has  so  far  shifted  its  position  as  to  cut  a  great 
meander  scarp  into  the  oldest  series  of  the  Swinemiinde  ridges, 
as  is  clearly  shown  by  the  map  (Fig.  132). 

Keilhack  has  made  a  careful  study  of  this  remarkable  series 
of  dune  ridges,  and  has  published  his  results  in  a  valuable  essay 
on  "  Der  Verlandung  der  Swinepforte."  He  found  the  distance 
between  ridge  crests  to  vary  from  130  to  150  feet  where  they 
were  closely  spaced,  and  from  330  to  460  feet  where  they  were 
farther  apart.  In  altitude  the  older  ridges  usually  do  not 
exceed  25  feet  absolute  elevation,  but  the  earliest  ridge  formed 
in  the  third  or  last  series  reaches  a  height  of  65  feet  or  more^*. 
Of  especial  interest  are  Keilhack's  observations  on  the  com- 
parative weathering  effects  in  the  three  systems  of  dune  ridges^^ 
The  dunes  of  the  latest  series  are  practically  unweathered  and 
retain  the  normal  light  color  of  the  beach  sands  from  which  they 
were  formed;  they  are,  therefore,  called  "  white  dunes."  Dunes 
of  the  next  older  series  show  a  thin  surface  layer  of  bleached 
sand,  below  which  the  sand  is  colored  yellow  by  limonite;  these 
are  known  as  the  "  yellow  dunes."  Finally,  the  oldest  dunes 
have  a  thin  surface  layer  of  humus  from  less  than  an  inch  to 
an  inch  or  more  in  thickness,  below  which  is  the  bleached  sand 
zone  from  1  to  1|  feet  thick.  Beneath  the  bleached  zone  the 
sand  grains  have  a  coating  of  brown  limonite,  and  may  even  be 
locally  cemented  by  this  material  into  a  soft  ferruginous  sand- 
stone. These  "  brown  dunes  "  must  have  existed  essentially 
as  we  find  them  for  a  long  period  of  time  in  order  to  experience 
svich  pronounced  weathering  effects.  The  formation  of  the 
bleached  zone  is  attributed  to  the  leaching  action  of  atmos- 
pheric waters  carrying  CO2  and  humus  acids,  by  means  of  which 
all  iron  is  removed  from  the  upper  foot  or  eighteen  inches 
of  each  dune  ridge.  How  clearly  the  white  surface  band  is 
contrasted  with  the  darker  sand  below  when  exposed  in  cross 
section,  may  be  seen  in  Plate  LH,  which  represents  a  road-cut 
through  an  old  dune  ridge  back  of  the  present  shore  at  Daytona, 


434  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 


RATE  OF  BEACH  RIDGE  FORMATION  435 

Florida.  The  weathering  phenomena  characteristic  of  the  yellow 
dunes  of  Keilhack  was  clearly  evident  in  the  older  dunes  of  the 
Darss,  but  I  saw  no  such  advanced  stages  of  alteration  as  that 
author  describes  for  his  brown  dunes.  I  am,  therefore,  inclined 
to  agree  with  Otto'*"  that  the  oldest  preserved  dune  ridges  of  the 
Darss  are  not  so  ancient  as  the  oldest  ridges  near  Swinemiinde. 
Perhaps  the  ridges  earliest  formed  near  the  base  of  the  Darss 
and  later  eroded-*^  were  more  closely  similar  to  the  brown  dune 
ridges  of  Keilhack. 

Through  a  comparison  of  reliable  maps  Keilhack  has  been 
able  to  show  that  between  the  year  1694  and  the  beginning  of 
the  twentieth  century  the  shore  west  of  the  northern  outlet  of 
the  Swine  was  prograded  nearly  one  mile  (1500  meters),  while 
elsewhere  the  advance  was  less  marked.  Since  1694  six  dune 
ridges  have  been  formed,  or  an  average  of  one  ridge  in  every 
35  years.  The  author  then  points  out  that  this  figure  agrees 
so  remarkably  with  the  figure  found  by  Bruckner  for  a  periodic 
climatic  oscillation,  that  one  cannot  well  refuse  to  accept  Sol- 
ger's  opinion  in  favor  of  a  genetic  connection  between  the 
formation  of  parallel  dune  ridges  and  this  climatic  period.  He, 
therefore,  accepts  35  years  as  the  time  represented  by  each 
ridge,  and  derives  a  chronology  for  the  entire  tombolo.  East  of 
the  Swine,  the  150  ridges  of  brown  dune  forming  the  original 
Misdroy  spit  would  require  5200  years;  the  40  ridges  of  yellow 
dunes  which  followed  would  demand  1400  years;  and  the  7  or  8 
ridges  of  white  dunes  about  300  years  additional;  making  a  total 
of  7000  years  for  the  entire  series  of  dune  ridges  on  the  Misdroy 
side  of  the  tombolo.  The  number  of  ridges  on  the  Swinemiinde 
side  is  much  less,  but  the  record  there  is  assumed  to  be  less 
complete. 

To  the  7000  years  derived  in  the  manner  above  indicated, 
Keilhack  would  add  an  unknown  number  of  years  representing 
two  erosion  periods  which  separated  the  three  systems  of*dune 
ridges.  The  evidence  for  two  erosion  periods,  distinct  from  the 
periods  of  prograding,  is  not  convincing,  and  Keilhack's  discus- 
sion of  this  question  does  not  appear  to  be  consistent.  To 
account  for  the  truncation  of  the  northern  ends  of  the  Swine- 
miinde brown  dunes  previous  to  the  formation  of  the  next 
following  series  of  yellow  dunes,  he  invokes  a  subsidence  of 
the  entire  district,  amounting  possibly  to  as  much  as  6  to  10 


436  SHORE  RIDGES  AND  THEIR   SIGNIFICANCE 

feet,  which  would  decrease  the  supply  of  marine  sand  for  dune 
building  and  favor  erosion*^;  at  the  end  of  the  erosion  period, 
estimated  as  1000  to  2000  years  in  length,  it  seems  that  re-ele- 
vation is  considered  a  probable  though  not  necessary  cause  of 
the  resumption  of  ridge  building  which  resulted  in  the  next 
series  of  yellow  dunes'*^.  Inasmuch  as  coastal  subsidence  is 
appealed  to  in  order  to  account  for  the  cessation  of  ridge  build- 
ing and  the  initiation  of  erosion  on  the  Swinemiinde  portion  of 
the  area,  it  would  seem  natural  to  expect  that  the  subsidence 
would  effect  the  same  changes  on  the  Misdroy  hook  just  across 
the  Swine.  But  since  the  Swinemiinde  hook  has  but  80  ridges, 
and  the  Misdroy  hook  150  ridges  in  the  oldest  series,  it  is  evi- 
dent that  according  to  Keilhack's  interpretation  the  Misdroy 
hook  must  have  continued  to  advance  for  2400  years  after  subsi- 
dence is  supposed  to  have  arrested  the  advance  of  the  Swine- 
miinde hook;  indeed,  Keilhack  specifically  states  that  the  ero- 
sion which  truncated  the  Swinemiinde  hook  may  very  well  have 
occurred  during  the  same  2400  years  that  the  Misdroy  hook  was 
still  advancing^",  apparently  not  realizing  that  this  invalidates 
his  previous  arguments  in  favor  of  repeated  depressions  and 
re-elevations  of  the  area  as  a  cause  of  alternate  periods  of  shore- 
line erosion  and  deposition.  To  account  for  the  cessation  of 
the  building  of  the  yellow  dunes,  their  truncation  by  erosion, 
and  the  later  building  of  the  white  dunes,  Keilhack  imagines  a 
second  movement  of  subsidence,  introducing  an  erosion  period 
some  hundreds  of  years  long,  followed  probably  by  a  slight 
elevation  which  occurred  between  1500  and  1600  A.D.  and  ex- 
posed great  masses  of  sand  on  a  wide  beach  which  the  wind 
could  build  into  the  especially  high  dune  ridge  which  marks  the 
beginning  of  the  white  dune  system^^ ;  but  on  the  following  page 
of  his  essay  he  states  that  the  eastern  half  of  the  Swinemiinde- 
Misdroy  region  continues  to  be  eroded  up  to  the  present  day. 
Thi^  this  author  invokes  coastal  subsidence  in  order  to  account 
for  shoreline  erosion,  yet  recognizes  such  erosion  following  coastal 
elevation. 

The  reasons  for  rejecting  the  oft-repeated  opinion  that  shore- 
line erosion  implies  coastal  subsidence  have  already  been  discussed 
at  some  length.  In  the  opinion  of  the  present  wi'iter  all  of  the 
phenomena  described  by  Keilhack  as  characteristic  of  the  Swine- 
pforte  dune  ridges  are  readily  to  be  explained  without  invoking 


RATE  OF  BEACH  RIDGE   FORMATION  437 

any  changes  in  relative  level  of  land  and  sea.  Beach  ridges  and 
dune  ridges  have  in  the  past  been  built  forward  at  one  place  and 
truncated  in  another  simultaneously,  just  as  the  Dungeness  is 
today  having  its  ridges  of  shingle  cut  away  on  the  south  side  and 
built  forward  on  the  east ;  or  as  Cape  Canaveral  is  being  eroded 
on  the  east,  and  prograded  on  the  south;  or,  indeed,  as  the  white 
dunes  near  Swinemiinde  have  been  built  forward  in  the  same 
time  that  the  closely  adjacent  coast  was  cut  bade.  In  fact,  the 
erosion  at  one  place  causes,  or  at  least  accelerates,  the  forward 
building  at  another  by  increasing  the  supply  of  shore  debris. 
It  is  to  be  expected  that  progressive  addition  of  beach  or  dune 
ridges  will  in  time  so  change  the  outline  of  the  shore  and  hence 
the  intensity  and  direction  of  marine  forces,  that  the  profile  of 
equilibrium  on  adjacent  parts  of  the  shore  will  be  disturbed,  and 
erosion  will  replace  deposition  at  certain  points,  without  any 
change  in  land  or  sea  level  and  without  any  profound  revolu- 
tion in  the  nature  of  the  marine  forces  operating  on  the  shore. 
The  equilibrium  of  a  shore  profile  is  a  very  delicate  thing,  and 
it  may  very  easily  be  so  disturbed  that  an  excess  of  erosion 
replaces  a  former  excess  of  deposition. 

It  is  highly  prol^able  that  much  if  not  all  of  the  erosion  of 
dune  ridges  which  occurred  in  the  Swinepforte  district  took 
place  while  dune  ridges  were  forming  in  other  parts  of  the  area; 
and  that,  therefore,  no  additional  time  is  to  be  allowed  for  these 
erosion  intervals.  Keilhack  recognized  the  uncertainty  of  the 
erosion  intervals,  and,  therefore,  permitted  his  estimate  of  7000 
years  to  remain  unchanged,  merely  stating  that  the  time  interval 
sidce  the  Litorina  submergence  which  introduced  the  period  of 
ridge  building  must  be  more  than  7000  years. 

We  may  accept  Keilhack's  estimate  of  35  years  as  the  average 
time  required  for  the  construction  of  each  of  the  six  ridges  of 
white  dunes  formed  since  1694,  without  agreeing  to  the  cor- 
relation of  dune  ridge  development  with  Bruckner's  climatic 
cycle,  or  to  the  proposed  chronology  of  the  older  dune  ridges. 
We  have  already  seen  that  the  physical  forces  which  control 
the  growth  of  successive  beach  and  dune  ridges  are  so  impor- 
tant in  magnitude  and  so  variable  in  their  activities  that  they 
would  scarcely  be  materially  affected  by  the  very  moderate 
climatic  changes  of  the  35  year  period.  It  is  true  that  Kriiger^- 
in  his  study  of   "  Sturmfluten  an   den  deutschcu    Klisten   der 


438  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 

westlichen  Ostsee  "  reaches  the  conclusion  that  periods  of  fre- 
quent "  storm  tides  "  alternate  with  periods  in  which  their 
occurrence  is  rare,  and  that  these  periods  correspond  in  a  gen- 
eral way  with  the  dry  and  wet  periods  respectively  of  the 
Bruckner  cycle.  There  are,  however,  striking  exceptions  to 
Kriiger's  rule  which  cast  some  doubt  on  its  value  and  certainly 
invalidate  it  for  use  in  establishing  a  beach  ridge  chronology. 
There  seems  to  be  no  escape  from  the  conclusion  that  the  supply 
of  sand,  the  intensity  and  frequency  of  great  storms,  the  length 
and  position  of  the  ridges,  and  other  controlling  factors  have 
varied  so  greatly  during  the  building  of  the  Swinemiinde-Misdroy 
tombolo  that  the  average  time  for  ridge  building  has  been  very 
different  at  different  places  and  at  different  periods.  Thus  it  is 
not  impossible  that  the  150  short  recurved  points  of  the  original 
Misdroy  spit  were  built  in  nearly  the  same  length  of  time  as 
the  80  longer  ridges  which  were  added  to  the  front  side  of  the 
Swinemiinde  spit,  even  though  the  cutting  of  the  meander 
scarp  in  the  Swinemiinde  series  suggests  that  the  Misdroy  spit 
may  have  added  a  few  of  its  recurved  points  after  the  Swine- 
miinde ridges  were  completed,  thereby  deflecting  the  Swine 
against  the  latter. 

The  Swine  has  built  a  beautiful  delta  into  the  Haff  south  of 
the  tombolo,  and  it  seems  probable  that  it  carries  an  appre- 
ciable amount  of  debris  into  the  bay  on  the  north  when  it  flows 
in  that  direction.  If  so,  wave  action  should  utilize  this  debris 
to  prograde  the  shore  with  unusual  rapidity  near  the  Swine 
mouth.  The  existence  of  moles  or  jetties  on  either  side  of  the 
mouth  may  also  tend  to  check  longshore  transportation  and  to 
accelerate  prograding  in  that  vicinity.  As  shown  by  Figure  132, 
there  is  a  delta-like  projection  of  the  dune  ridge  series  at  the 
mouth  of  the  Swine,  where  formerly  an  embayment  existed  as 
shown  by  the  older  ridges;  and  Keilhack  states  that  the  moles 
at  the  mouth  of  the  Swine  have  made  the  shore  build  forward 
there  much  more  rapidly  than  usual  during  the  last  two  cen- 
turies^^. The  six  dune  ridges  built  within  this  same  period,  and 
used  by  Keilhack  as  a  basis  for  his  calculations,  may,  therefore, 
represent  a  much  smaller  time  interval  than  six  ridges  of  the 
older  series.  Many  of  the  latter  may  have  required  an  average 
of  50  years  or  more  for  the  construction  of  each  ridge. 

Enough  evidence  has  been  presented  to  show  the  impossi- 


BEACH  RIDGES  AS  RECORDS  OF  CHANGES   OF  LEVEL     439 

bility  of  building  up  any  accurate  chronology  on  the  basis  of 
beach  ridges  or  dune  ridges.  On  the  other  hand,  it  appears  that 
a  large  series  of  extensive  ridges  must  represent  a  long  time  in- 
terval, and  that  25  to  50  years  is  not  an  improbable  figure  for 
the  time  required  to  build  such  prominent  ridges  as  are  character- 
istic of  the  Dungeness,  Darss,  and  Swinepforte  areas.  Beach 
and  dune  ridges,  therefore,  have  a  great  value  in  acquainting 
us  with  the  order  of  magnitude  of  the  minimum  time  involved 
in  their  construction,  even  though  they  cannot  furnish  more 
precise  data. 

Beach  Ridges  as  Records  of  Changes  of  Level.  —  A  well- 
developed  series  of  beach  ridges  may  have  a  high  value  as  evi- 
dence of  former  changes  in  relative  level  of  land  and  sea,  or  of 
coastal  stability.  If  there  is  a  gradual  emergence  of  the  land 
during  the  development  of  the  ridges,  it  would  seem  that  the 
crests  of  older  members  of  the  series  should  be  found  at  pro- 
gressively higher  elevations  above  water  level;  whereas  con- 
tinued submergence  should  be  indicated  by  a  decrease  in  crest 
altitude  as  one  passes  inland  from  the  modern  ridges.  Coastal 
stability,  on  the  other  hand,  should  be  recorded  by  a  general 
agreement  of  ridge  crest  altitude  throughout  the  series. 

If  applied  with  discrimination  and  with  a  full  understanding 
of  the  different  conditions  which  determine  the  altitudes  of 
beach  ridges,  the  above  principle  may  throw  valuable  light  on 
the  interesting  questions  relating  to  past  changes  in  the  level  of 
land  and  sea.  Its  indiscriminate  and  uncritical  use  will  often 
lead  to  erroneous  conclusions.  It  behooves  us,  therefore,  to 
take  cognizance  of  certain  fundamental  facts  concerning  the 
formation  of  beach  ridges,  and  to  note  in  what  ways  they  may 
affect  our  judgment  in  interpreting  the  significance  of  crest 
altitudes.  Again  it  will  be  most  convenient  to  state  the  facts 
categorically,  and  comment  on  them  as  may  seem  desirable. 

1.  The  terminal  points  of  recurved  spits  normally  descend 
toward  their  distal  ends  and  pass  under  the  water  level,  as  has 
previously  been  shown.  In  a  compound  recurved  spit,  there- 
fore, it  will  often  be  found  that  the  ridges  back  of  the  present 
shore,  representing  successive  recurved  points,  are  materially 
lower  than  the  modern  beach  ridge.  It  is  difficult  to  see  how 
any  one  could  regard  such  difference  in  ridge  crest  elevation  as 
an  evidence  of  coastal  subsidence;    yet  it  has  been  so  regarded 


440  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 

by  several  observers.  One  should  clearly  realize,  however,  that 
the  point  of  a  spit  which  curves  back  into  more  protected  and 
quieter  waters  thereby  escapes  that  direct  impact  of  the  larger 
waves  which  is  necessary  to  heap  up  the  sand  or  gravel  to  the 
greatest  altitudes;  and  that  the  failure  of  an  adequate  supply 
of  debris  near  the  terminus  necessitates  a  low  embankment  in 
any  case.  Observation  will  suffice  to  show  that  where  the 
points  of  such  spits  are  lengthening  from  year  to  year,  they  are 
built  low  in  the  first  place,  and  do  not  acquire  their  low  level 
by  subsidence. 

2.  Beach  and  dune  ridges  of  great  linear  extent  normally 
vary  in  altitude  along  their  crests.  If  they  possess  free  ends, 
they  usually  descend  more  or  less  gradually  and  pass  under  the 
water;  for  while  they  may  not  recurve  into  quieter  water,  such 
unattached  ridge  ends  resemble  spits  to  the  extent  that  the 
supply  of  debris  at  their  distal  points  is  insufficient  to  build  up 
a  submarine  embankment  and  raise  it  to  a  considerable  eleva- 
tion above  sealevel.  It  is  not  necessary,  for  example,  to  regard 
the  descending  southern  end  of  the  oldest  Swinemiinde  ridges^* 
as  an  evidence  of  coastal  submergence.  Variations  in  supply 
of  material,  in  exposure  to  wave  action,  in  depth  of  offshore 
bottom,  and  in  other  factors  may  cause  a  marked  variation  in 
crest  altitude  anywhere  along  the  course  of  the  dunes.  On 
the  Darss  foreland,  where  observations  indicate  long-continued 
coastal  stability,  a  large  number  of  the  older  east-west  dune 
ridges  are  low  in  the  central  part  and  high  at  either  end. 

3.  Successive  beach  and  dune  ridges  normally  differ  from 
each  other  in  altitude  of  crest  line.  This  follows  from  what 
has  already  been  said  regarding  the  origin  of  such  ridges.  A 
temporary  excess  of  shore  debris  may  cause  a  new  ridge  to 
form  before  the  earlier  one  behind  it  had  acquired  any  consid- 
erable altitude;  and  the  new  ridge  may  rise  to  a  great  height 
before  the  development  of  a  still  later  ridge  checks  its  growth. 
Temporary  retrograding  of  the  shoreline  may  combine  several 
low  ridges  into  one  high  one,  while  earlier  and  later  ridges  re- 
main of  moderate  altitude.  The  great  variability  of  the  marine 
forces  causes  the  successive  positions  of  the  shoreline  to  be 
maintained  for  unequal  lengths  of  time,  and  to  have  unequal 
quantities  of  shore  debris  cast  into  shore  ridges  of  unequal 
height.     It  may  happen   that  one  ridge  is  not  raised   above 


BEACH  RIDGES  AS  RECORDS   OF   CHANGES  OF  LEVEL     441 


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442  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 

water  level  before  another  is  built  in  front  of  it,  but  as  a  rule 
the  differences  in  height  are  all  to  be  measured  above  the  level 
of  the  sea.  Beach  ridges  are  formed  directly  by  the  waves,  and 
cannot,  of  course,  exceed  the  height  to  which  waves  in  a  given 
exposure  are  capable  of  raising  the  material  of  which  the  ridges 
are  composed.  This  may  be  only  three  or  four  feet  in  a  sheltered 
locality,  but  very  commonly  amounts  to  10  or  15  feet  for  ordi- 
nary shingle  beach  ridges  on  an  open  coast,  and  a  single  semi- 
permanent ridge  like  the  Chesil  Bank  on  the  exposed  south 
coast  of  England  may  reach  a  height  of  40  to  50  feet  above 
high  water^^.  It  is  no  uncommon  thing  to  find  beach  ridges  10 
feet  or  more  in  height  irregularly  interspersed  with  others  less 
than  half  as  high;  and  theoretically  the  difference  may  be  as 
great  as,  or  greater  than,  the  maximum  height  of  the  ridges 
above  water  level.  Practically,  however,  the  irregular  varia- 
tions in  crest  altitude  are  commonly  not  much  greater  than  half 
the  altitude  of  the  higher  ridges,  and  in  many  cases  are  appre- 
ciably less.  Goldthwait^^  has  described  a  case  in  which  a  series 
of  sixteen  consecutive  ridges  having  an  average  crestline  alti- 
tude of  3.82  feet  above  high  water  contained  no  ridge  higher 
than  4.64  feet  nor  any  lower  than  3.04  feet;  an  extreme  differ- 
ence of  bv.t  1|  feet. 

Dune  ridges  owe  their  height  to  the  action  of  the  wind,  and 
may,  therefore,  rise  well  above  the  upper  reach  of  storm  waves 
for  a  given  exposure.  Dune  ridges  65  feet  high  are  known"  on 
the  shores  of  the  Baltic,  and  the  remarkable  height  of  nearly 
300  feet  is  reported  from  the  somewhat  irregular  dune  ridges 
along  the  coast  of  the  Landes  in  France^^.  Usually,  however, 
15  to  25  feet  is  the  upper  limit  for  individual  members  of  an 
extensive  series  of  dune  ridges.  As  is  to  be  expected,  variations 
in  altitude  among  different  ridges  of  a  given  dune  system  are 
greater  than  in  the  case  of  beach  ridges,  because  to  the  variable 
factors  affecting  the  initial  beach  ridge  upon  which  the  dunes 
stand  are  added  the  variable  factors  which  affect  dune  accumu- 
lation, including  strength  and  direction  of  the  winds  which  move 
the  dry  sands  of  the  upper  part  of  the  beach,  and  the  character 
of  the  dune  vegetation.  Twelve  dune  ridges,  associated  with 
the  very  accordant  beach  ridges  described  by  Goldthwait  and 
mentioned  above,  were  found  by  that  author  to  vary  from  3.91 
to  9.04  feet  in  height  above  high  water^^     Dune  ridges  on  the 


BEACH  RIDGES  AS  RECORDS  OF  CHA.NGES  OF  LEVEL     443 

Darss  vary  in  altitude  from  2  feet  or  less  to  25  feet  or  more, 
measured  from  the  bottom  of  adjacent  swales.  Among  the 
Swinemiinde  dune  ridges  are  many  3  to  6  feet  high,  others  25 
feet  or  more,  and  one  or  two  as  high  as  65  feet  above  sealevel. 
The  Cape  Canaveral  dune  ridges  vary  from  2  to  12  feet  above 
high  tide  level.  Perhaps  the  lowest  dune  ridges  are  merely 
beach  ridges  with  the  surface  sands  slightly  disturbed  by  the 
w^ind. 

It  is  manifestly  impossible  to  regard  such  variations  in  the 
level  of  individual  beach  ridge  and  dune  ridge  crests  as  indica- 
tions of  elevations  and  subsidences  of  the  land.  Probably  no 
one  would  be  so  bold  as  to  imagine  such  a  rapid  and  oft-repeated 
alternate  rising  and  falling  of  the  coast  as  would  be  called  for  by 
the  great  series  of  ridges  of  the  Dungeness,  Darss,  Swinemiinde, 
and  Canaveral  beach  plains,  were  variations  in  ridge  height  to 
be  regarded  as  proving  variations  in  sealevel.  It  follows  that 
one  should  be  equally  cautious  in  accepting  the  inequality  in 
height  of  two  or  three  ridges  as  a  proof  of  changes  of  level;  for 
if  many  ridges  may  acquire  unequal  altitudes  without  the  aid 
of  vertical  movements  of  land  or  sea,  certainly  a  few  may  do  so. 
If  we  are  satisfied  as  to  the  validity  of  this  conclusion,  we  shall 
have  no  difficulty  in  realizing  the  fallacy  of  one  of  the  lines  of 
argument  not  infrequently  advanced  in  support  of  theories  of 
coastal  subsidence  and  elevation. 

4.  In  a  given  series  of  beach  or  dune  ridges  there  is  a  tendency 
for  those  first  formed  to  have  a  lower  altitude  than  later  members 
of  the  series.  Cornish'''^  was  of  the  opinion  that  "it  is  unneces- 
sary to  invoke  upheaval  or  subsidence  to  account  for  such 
difference  of  level,"  and  explained  the  greater  height  of  the 
later  ridges  on  the  ground  that  as  a  foreland  builds  farther  and 
farther  out  into  the  water  it  offers  increased  obstruction  to  the 
coastal  currents,  thus  causing  them  to  bank  up  the  water  to  a 
greater  height  and  raising  the  level  of  ridge  construction.  We 
may  agree  with  Cornish's  general  conclusion,  yet  doubt  whether 
the  level  of  the  water  is  ever  sufficiently  affected  by  foreland 
growth  to  account  for  the  phenomena  in  question.  It  is  evi- 
dent, however,  that  on  a  sloping  bottom  only  small  waves  can 
operate  near  the  shore,  since  waves  break  when  entering  water 
of  a  depth  about  equal  to  their  height.  A  beach  ridge  built 
near  the  shore  will   tend   to   have   a   low   altitude,   for   small 


444  SHORE  RIDGES  AND   THEIR  SIGNIFICANCE 

waves  cannot  cast  debris  to  a  great  elevation.  Waves  of  greater 
height,  breaking  farther  seaward,  finally  build  a  new  ridge  in 
front  of  the  one  first  formed,  and  are  able  to  build  the  crest  of 
this  new  member  of  the  series  to  a  somewhat  greater  altitude 
than  that  of  its  predecessor.  Still  larger  storm  waves  may 
build  a  third  ridge  of  still  greater  height;  and  in  this  manner 
there  is  produced,  as  the  result  of  normal  wave  action  on  a 
stable  coast,  a  series  of  beach  ridges  of  increasing  altitude  going 
seaward.  There  can  be  no  doubt  that  this  history  of  beach 
ridge  development  has  been  repeated  in  many  places  along  our 
coasts,  and  it  is,  therefore,  manifestly  impossible  to  regard  a 
landward  decrease  in  beach  crest  altitude,  especially  in  a  series 
of  a  few  ridges  only,  as  a  proof  of  coastal  subsidence. 

During  the  early  stages  of  beach  ridge  formation  on  a  shelving 
sea-bottom,  it  is  probable  that  the  zone  of  ridge  building  is 
shifted  seaward  with  constantly  diminishing  rapidity.  The 
first  ridge  is  quickly  built  by  the  smaller  waves.  Soon  larger 
wares  begin  the  construction  of  a  new  ridge  in  front  of  the  first. 
The  shoreline  remains  for  a  longer  time  in  this  new  position, 
because  no  change  will  occur  until  the  waves  have  built  up  from 
a  deeper  sea-bottom  a  ridge  of  sufficient  height  to  transfer  the 
shore  activities  permanently  to  a  third  position  still  farther 
seaward.  We  have  already  seen  that  the  longer  a  shoreline 
remains  in  a  given  position,  the  greater  is  the  probability  that 
the  shore  ridge  will  be  raised  to  a  high  elevation.  On  this 
account  we  may  reasonably  suppose  that  progressively  slower 
advance  of  a  shoreline  often  helps  to  produce  a  series  of  beach 
ridges  whose  crest  altitudes  decrease  in  a  landward  direction. 

A  further  cause  of  normal  decrease  in  altitude  of  progressively 
older  beach  ridges  is  probably  to  be  found  in  the  greater  weather- 
ing to  which  the  older  members  of  the  series  have  been  subjected. 
In  the  course  of  many  centuries  it  seems  certain  that  a  ridge  of 
gravel  loosely  piled  up  by  the  waves  must  become  somewhat 
compacted;  while  sand  ridges  will  be  very  slowly  worn  lower 
under  the  constant  attack  of  rains  and  other  agencies  of  weather- 
ing. It  can  hardly  be  supposed  that  in  the  course  of  a  few 
thousand  years  such  changes  in  crest  altitude  would  be  very 
pronounced;  but  we  may  fairly  assume  that  they  would  be 
appreciable,  and  might  therefore  serve  to  augment  similar  dif- 
ferences due  to  other  causes. 


BEACH   RIDGES  AS   RECORDS  OF   CHANGES  OF  LEVEL     445 

Dune  ridges  formed  on  low  beach  ridges,  and  dune  ridges  that 
have  had  only  a  short  time  in  which  to  accumulate  by  reason 
of  a  rapid  prograding  of  the  shore,  or  that  have  been  acted 
upon  by  the  weather  for  thousands  of  years,  tend  to  have  a  low 
crest  altitude.  From  what  has  been  said  regarding  beach 
ridges  it  follows,  therefore,  that  successive  dune  ridges  with 
diminishing  crest  heights  going  inland  may  be  a  normal  feature 
of  a  stable  coast,  and  that  they  are  no  more  to  be  regarded  as 
proofs  of  coastal  subsidence  than  are  beach  ridges  showing 
similar  relations  of  crest  lines. 

It  should  be  fully  understood  that  beach  and  dune  ridges  of 
progressively  decreasing  altitude  landward  are  normal,  but  by 
no  means  necessary,  features  of  a  prograding  shore.  They  are 
more  apt  to  characterize  the  earliest  stages  of  shore  prograding, 
and  we  must  be  prepared  to  find  the  oldest  members  of  a  large 
series  of  ridges,  or  all  the  members  of  a  very  small  series,  showing 
the  phenomenon  in  question  at  various  places  along  a  shore 
which  has  experienced  no  change  of  level  since  ridge  building 
began.  On  the  other  hand,  the  conditions  which  determine 
shoreline  development  are  so  complex  and  are  subject  to  such 
variations  that  one  cannot  expect  to  find  a  simple,  regular  de- 
crease in  crest  altitude  as  a  common  feature  of  all  beach  and 
dune  ridge  series.  On  the  contrary,  it  is  only  under  favorable 
conditions  that  the  tendency  to  produce  such  regular  differences 
in  altitude  is  not  masked  or  completely  overcome  by  other 
forces.  We  shall  find  instances  in  which  rapid  prograding  of 
the  shore  has  produced  a  series  of  low  ridges  next  the  present 
shoreline,  while  older  ridges  have  a  higher  average  elevation. 

5.  Beach  ridges  are  more  valuable  than  dune  ridges  in  deter- 
mining changes  of  level  or  coastal  stability.  This  follows  from 
the  fact  that  dune  ridges  show  greater  local  variations  in  height 
than  do  beach  ridges,  as  is  explained  in  paragraph  No.  3  above. 
It  is  clear  that  safe  conclusions  as  to  past  moderate  variations 
of  relative  sealevel  or  past  coastal  stability  cannot  be  so  readily 
based  upon  ridges  which  may  show  wide  differences  of  level  due 
to  causes  independent  of  vertical  changes  in  the  position  of  land 
or  sea,  as  they  can  upon  ridges  which  normally  vary  within 
much  narrower  limits. 

6.  Beach  ridges  and  dune  ridges  must  be  regarded  as  incom- 
parable features,  when  one  is  seeking  to  determine  the  possi- 


446  SHORE  RIDGES  AND   THEIR  SIGNIFICANCE 

bility  of  past  changes  of  level.  It  is  not  permissible  to  compare 
an  older  series  of  beach  ridges  with  a  later  series  of  dune  ridges, 
or  vice  versa,  and  to  infer  subsidence  or  elevation  if  the  average 
heights  of  the  two  dissimilar  series  are  unlike.  The  force  which 
predominates  in  dune  building  is  not  the  same  as  the  force  which 
predominates  in  beach  building,  and  there  is  no  reason  why  the 
two  forces  should  build  ridges  of  similar  height.  On  the  con- 
trary, dune  ridges  are  built  upon  pre-existing  beach  ridges,  and 
must,  therefore,  exceed  the  latter  in  altitude.  Near  Sandham- 
maren  on  the  southern  coast  of  Sweden  a  magnificent  series  of 
beach  ridges  is  bordered  seaward  by  a  higher  series  of  dune 
ridges;  but  the  theory  that  this  part  of  the  Swedish  coast  is 
subsiding  is  disproved  by  evidence  which  I  will  present  in  an- 
other connection.  A  similar  relation  of  beach  and  dune  ridges 
on  the  coast  of  eastern  Canada  has  been  cited  by  Ganong  as  an 
evidence  of  coastal  subsidence,  although  Goldthwaif'i  has  shown 
that  the  later  and  higher  dune  ridges  of  this  region  rest  upon 
gravel  beach  ridges  of  the  same  height  as  the  older  beach  ridges. 
Beach  ridges  may  properly  be  compared  as  to  altitude  with  other 


Fig.  133.  —  Beach  ridges  indicating  coastal  emergence. 

beach  ridges  underlying  dune  ridges,  when  their,  surfaces  are 
sufficiently  exposed  for  this  purpose;  but  never  with  the  super- 
imposed dune  ridges  themselves. 

7.  Both  beach  ridges  and  dune  ridges  have  a  distinct  value 
as  records  of  changes  of  level  or  of  coastal  stability,  notwith- 
standing the  restrictions  mentioned  above.  A  large  series  of 
beach  ridges  which  may  show  irregular  variations  in  heights  of 
individual  crests  but  which  is  characterized  in  addition  by  a 
gradual  landward  increase  in  the  average  height  of  the  ridges, 
or  in  the  heights  of  the  principal  ridges  (Fig.  133),  is  strongly 
suggestive  of  emergence.  If  the  seaward  members  of  the  series 
have  a  considerable  height,  indicating  that  they  have  more  or 
less  nearly  attained  the  maximum  elevation  which  waves  in 
that  exposure  can  give  to  ridges,  while  the  older  ridges  have  a 


BEACH  RIDGES  AS  RECORDS  OF  CHANGES  OF  LEVEL     447 

much  greater  height,  the  evidence  may  be  said  to  furnish  satis- 
factory proof  of  elevation.  I  have  found  such  ridges  on  the 
northern  shores  of  the  Baltic  Sea,  where  independent  evidence 
indicates  progressive  emergence  of  the  land.  Care  should  be 
taken,  however,  in  employing  this  line  of  evidence  in  those  cases 
where  the  prograding  of  the  shore  materially  reduces  the  width 
of  the  water  body  upon  which  the  ridge-making  waves  are  de- 
veloped; for  the  size  of  the  waves  will  thereby  be  reduced,  and 
the  seaward  members  of  the  ridge  series  will  decrease  in  alti- 
tude independently  of  coastal  emergence. 

We  have  already  seen  that  a  few  beach  ridges  exhibiting  a 
landward  decrease  in  crest  altitude  is  a  normal  feature  of  a 
stable  shore,  and  therefore  must  not  be  regarded  as  an  evidence 
of  coastal  submergence.  It  is  even  probable  that  a  large  series 
of  such  ridges  may  be  characterized  by  the  same  landward 
decrease  of  average  crest  altitude,  due  to  a  gradual  seaward 
increase  in  depth  of  water  and  size  of  waves,  and  to  other  factors 
favoring  greater  crest  height  in  the  later  ridges.  If  an  exten- 
sive series  of  beach  ridges  descending  landward  could  be  traced 
to  a  considerable  depth  beneath  the  surface  of  a  salt-marsh 
peat  deposit  composed  of  high  tide  vegetation  onl}^  which  had 
protected  the  ridges  from  destruction  by  extending  over  them 
as   they  sank  lower  and    lower    (Fig.    134),    coastal    submerg- 


FiG.   134.  —  Beach  ridges  indicating  coastal  submergence. 

ence  could  be  inferred  with  reasonable  certainty.  A  few  ridges 
at  a  shallow  depth  in  the  marsh  would  not  be  satisfactory  evi- 
dence; for  normal  wave  action  on  a  stable  shore  might  fail 
to  raise  the  initial  ridges  above  sealevel,  while  marsh  deposits 
might  later  protect  them  from  destruction  by  the  lagoon  waves. 
It  is  very  seldom  that  the  conditions  which  render  it  safe  to 
employ  beach  ridges  as  an  evidence  of  coastal  submergence 
exist.  In  all  of  the  cases  which  have  come  to  my  attention 
where  a  landward  decrease  in  ridge  crest  height  has  been  used 
as  a  proof  of  submergence,  such  use  has  not  seemed  to  me  justi- 
fiable, for  the  reason  that  the  phenomena  described  might 
equally  well  be  explained  as  the  normal  product  of  wave  action 


448 


SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 


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BEACH  RIDGES  AS  RECORDS  OF  CHANGES  OF  LEVEL  449 

on  a  stable  coast.  It  may  reasonably  be  doubted  whether 
beach  ridge  development  often  takes  place  on  a  subsiding  coast, 
since  subsidence  favors  marine  erosion,  and  is  highly  unfavorable 
to  the  prograding  of  shorelines. 

Where  a  large  series  of  beach  ridges  show  throughout  about 
the  same  average  crest  altitude,  or  about  the  same  altitude  for 
the  principal  ridges,  coastal  stability  is  strongly  indicated.  If 
the  older  and  later  ridges  are  both  about  as  high  as  the  present 
waves  could  be  expected  to  build  them,  the  evidence  in  favor  of 
long  continued  stability  may  be  regarded  as  conclusive.  There 
are  two  hypothetical  cases  which  might  lead  to  an  erroneous 
conclusion,  but  it  is  probable  that  danger  of  error  from  this 
source  would  be  eliminated  by  careful  observation.  One  may 
imagine  that  on  a  rising  coast  where  the  earliest  ridges  are  of 
small  altitude  and  the  later  ridges  progressively  higher,  the 
amount  of  elevation  might  just  be  sufficient  to  raise  the  crests 
of  the  first  ridges  into  the  same  horizontal  plane  with  the  crests 


Fig.  135.  —  Hypothetical  case  in  which  beach  ridges  on  a  rising  coast  may 
give  a  false  indication  of  stability. 

of  those  formed  later  (Fig.  135);  and  a  careless  observer  might 
argue  in  favor  of  coastal  stability  because  of  the  resulting  equal- 
ity of  crest  heights.  But  since  we  are  not  apt  to  find  high  beach 
ridges  with  very  narrow  bases,  while  the  low  ridges  formed  in 
shallow  water  are  characteristically  narrow,  comparison  of  the 
older  and  later  ridges  formed  in  the  manner  indicated  should 
reveal  the  fact  that  those  first  formed  are  really  low  ridges 
raised  high  above  the  plane  in  which  they  must  originally  have 
been  constructed.  This  is  made  clear  by  Figure  135.  The  case 
is  improbable,  not  merely  because  the  rate  of  emergence  must  be 
just  enough  to  give  the  required  equality  of  crest  altitudes,  but 
also  because  a  progressively  emerging  shore  favors  the  repeated 
development  of  small  ridges  rather  than  ridges  of  constantly 
increasing  height. 

A  second  case  may  be  imagined  in  which  progressive  sub- 
mergence carries  the  crests  of  older,  high  ridges  nearer  to  water 


450  SHORE   RIDGES  AND   THEIR  SIGNIFICANCE 

level,  thereby  bringing  them  into  the  same  horizontal  plane  as  the 
crests  of  successively  lower  ridges  formed  later.  Thus,  as  shown 
in  Figure  136,  one  might  infer  coastal  stability  from  equality 
of  ridge  crest  altitude  in  a  region  which  had  reaUy  experienced 
progressive  submergence.  The  true  history  might  be  suspected 
from  the  fact  that  an  increasing  proportion  of  the  older  larger 
ridges  was  below  marsh  level  or  the  level  of  lagoons  caused  by 


Fig.  136.  —  Hypothetical  case  in  which  beach  ridges  on  a  sinking  coast 
give  a  false  indication  of  stabiUty. 

the  submergence.  This  hypothetical  case,  is,  however,  even 
more  improbable  than  the  one  supposed  above,  since  it  involves 
not  only  a  special  rate  of  subsidence  and  the  building  of  the 
largest  ridges  in  the  shallowest  water  where  only  small  ridges 
are  to  be  expected,  but  also  because  submergence  tends  to  pre- 
vent ridge  building  entirely  and  to  favor  the  erosion  of  the  coast. 
As  shown  by  the  figure,  the  formation  of  the  smaller  ridges 
demands  an  increasingly  extensive  aggrading  of  the  deeper  off- 
shore bottom,  a  process  to  which  submergence  is  distinctly 
unfavorable. 

Widely  spaced  older  beach  ridges  rising  above  marsh  level 
back  of  a  later  series,  thereby  giving  a  superficial  appearance  of 
the  conditions  represented  in  Figure  136  must  not  be  regarded 
as  an  indication  of  subsidence,  since  such  ridges  may  have  been 
formed  with  wide  spaces  of  water  between  them  in  the  first 
mstance,  and  the  lagoons  converted  into  marshes  at  a  later  date. 
Several  ideal  profiles  through  beach  and  dune  ridge  series  formed 
on  stable  coasts  are  shown  in  Figure  137. 

Two  ridges  (Fig.  137  c)  of  similar  altitude  may  be  sufficient 
to  prove  long  continued  coastal  stability,  providing  they  are  so 
high  as  to  preclude  the  possibility  that  the  earliest  one  was  built 
much  higher  and  later  carried  down  by  subsidence,  and  providing 
also  the  older  one  is  manifestly  not  a  small  initial  ridge  raised  to 
its  present  height  by  coastal  elevation.  In  addition,  there  must 
be  some  means  of  proving  the  lapse  of  a  long  interval  of  time 
between  the  building  of  the  two  ridges.     A  case  of  this  kind  is 


BEACH  RIDGES  AS  RECORDS  OF  CHANGES  OF  LEVEL     451 

presented   by  Nantasket  Beach,  Massachusetts,  and  has  been 
fully  described  by  Johnson  and  Reed.®^ 

An  extensive  series  of  dune  ridges  may  furnish  reliable  evi- 
dence of  essential  coastal  stability,  if  their  formation  has  evi- 
dently required  so  long  a  period  of  time  that  any  marked  change 


Fig.  137.  —  Types  of  beach  ridges  formed  on  a  stable  coast. 

(a)  Earliest  beach  ridges  lower  because  of  shallow  water  nearest  the  original 

shoreUne. 
(h)  Similar  to  a,  but  older  ridges  isolated  in  marsh. 

(c)  Central  ridges  low  because  of  rapid  prograding  to  present  zone  of  wave 

action,  where  the  tendency  to  prograde  is  much  less  pronounced. 

(d)  Later  ridges  with  greater  average  height  than  older,  because  former  are 

dune  ridges  surmounting  beach  ridges,  whUe  latter  are  unmodified 
beach  ridges. 


of  level  must  of  necessity  have  resulted  in  a  pronounced  differ- 
ence in  crest  heights  recognizable  in  spite  of  individual  varia- 
tions in  ridge  altitude.  For  example,  if  the  members  of  an 
extensive  system  of  dune  ridges  vary  in  original  height  from  3 
to  25  feet,  with  the  exception  of  occasional  abnormal  individ- 
uals which  are  manifestly  the  product  of  special  conditions  and 
which  may  therefore  be  ignored;  and  if  the  average  height  of 
the  older  and  later  ridges  is  similar,  and  the  building  of  the  entire 
series  required  5000  years;  then  one  may  safely  reject  a  theory 
which   would   demand,  for   example,   a   continuous   progressive 


452 


SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 


subsidence  averaging  6  inches  or  a  foot  per  century.  For  a 
subsidence  at  the  smaller  rate  for  the  period  mentioned  would 
carry  the  highest  of  the  older  ridges  down  to  sealevel  and  would 
deeply  submerge  the  smaller  ones.  The  fact  that  there  has 
been  no  material  change  in  the  relation  of  dune  crests  to  sea- 
level  between  the  earlier  and  later  portions  of  the  series  is  suffi- 
cient indication  that  there  has  been  no  marked  change  in  the 
relative  level  of  land  and  sea.  To  admit  the  possibility  of  pro- 
gressive subsidence  of  the  land,  we  would  have  to  assume  that 
prograding  took  place  in  spite  of  subsidence,  that  the  earliest 
formed  ridges  were  built  25  feet  higher,  on  an  average,  than 


Fig.  138.  —  Beach  ridges  of  equal  height  separated  by  swales  of  different 
depthsTdue  to'variations  in  spacing  of  ridges. 

the  modern  ones,  and  that  this  excess  of  height  decreased  with 
some  degree  of  regularity  and  at  about  the  same  rate  as  subsi- 
dence carried  the  land  downward;  a  series  of  assumptions  diffi- 
cult to  grant. 

8.  The  levels  of  swale  bottoms,  whether  between  beach  ridges 
or  dune  ridges,  is  of  comparatively  little  significance.  This 
follows  from  the  fact  that  the  depth  of  the  swales  depends  in 
large  measure  upon  the  closeness  of  the  spacing  of  the  ridges, 
which  is  in  turn  dependent  upon  factors  not  usually  related  to 
changes  of  level.  Figure  138  will  serve  to  make  clear  the  fact 
that  a  series  of  similar  ridges  of  equal  height,  built  on  a  stable 
shore  by  a  prograding  process  which  varied  in  rate  with  varia- 
tions in  supply  of  debris  by  longshore  currents,  may  be  separ- 
ated by  swales  of  very  unequal  depth. 


RESUME 


We  have  inquired  into  the  origin  of  beach  ridges  and  dune 
ridges  and  have  found  that  while  they  are  produced  by  waves 
operating  under  a  variety  of  circumstances,  they  are  not  to  be 
correlated  with  individual  great  storms.     Among  the  types  of 


REFERENCES  453 

current  action  responsible  for  the  supply  of  debris  Imilt  into 
parallel  ridges,  longshore  beach  drifting  resulting  from  waves 
breaking  obliquely  on  the  shore,  although  too  commonly  neglected, 
is  believed  to  be  one  of  the  most  important.  The  conditions 
which  control  the  heights  of  beach  and  dune  ridges  have  been 
discussed  at  length,  as  have  also  the  conditions  affecting  the  rate 
of  ridge  development.  For  our  guidance  in  attempting  to  esti- 
mate the  approximate  time  represented  by  any  given  series  of 
beach  ridges  or  dune  ridges,  certain  general  principles  have  been 
laid  down;  and  an  examination  of  the  known  or  estimated  rates 
of  ridge  formation  on  certain  important  beach  plains  has  pro- 
vided data  which  will  be  of  some  service  in  making  such  attempts. 
Finally,  it  has  been  shown  that,  when  interpreted  with  caution, 
beach  and  dune  ridges  may  furnish  valuable  evidence  as  to  past 
changes  in  the  relative  level  of  land  and  sea;  and  a  series  of 
eight  fundamental  principles,  the  recognition  of  which  is  essen- 
tial to  a  proper  interpretation  of  such  evidence,  has  been  pre- 
sented and  discussed. 

REFERENCES 

1.  Redman,  J.  B.     The  East  Coast  between  the  Thames  and  the  Wash 

Estuaries.     Min.  Proc.  Inst.  Civ.  Eng.     XXIII,  186-256,  1864. 

2.  Drew,  F.     [On  the  Dungeness]  quoted  by  Wm.  Topley  in  "The  Geology 

of  the  Weald."     Mem.  Geol.  Surv.  England  and  Wales,  pp.  212-215, 
302-312,  1875. 

3.  Redman,  J.  B.     On  the  Alluvial  Formations,  and  the  Local  Changes 

of  the  South  Coast  of  England.    Min.  Proc.  Inst.  Civ.  Engr.    XI,  162- 
204,  1852. 
4    Gulliver,    F.    P.     Dungeness   Foreland.     Geog.   Jour.,    London.      IX, 
536-546,  1897. 

5.  Otto,  Theodor.      Der  Darss  und  Zingst.      Jahresb.  der  Geog.  Gesell. 

Greifswald.     XIII,  235-485,  1913. 

6.  Keilhack,  K.     Die  Verlandung  der  Swinepforte.     Jahrbuch  der  Konigl. 

Preuss.  Geo!.  Landesanstalt  f.  1911.     XXXII,  209-244,  1912. 

7.  Gilbert,  G.  K.     Lake  Bonneville.     U.  S.  Geol.  Surv.,  Mon.     I,  47,  55, 

1890. 

8.  lUd.,  pp.  56-57. 

9.  Davis,  W.  M.     Die  Erklarende  Beschreibimg  der  Landformen,  p.  473, 

Leipzig  and  Berlin,  1912. 

10.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

pp.  708-709,  Boston,  1909. 

11.  Ibid.,  pp.  710-715. 

12.  Gilbert,  G.  K.     Lake  Bonneville.     U.  S.  Geol.  Surv.,  Mon.     I,  57,  1890. 

13.  Davis,  W.  M.     Geographical  Essays.     Edited  by  Douglas  W.  Johnson, 

p.  709,  Boston,  1909. 


454  SHORE  RIDGES  AND  THEIR  SIGNIFICANCE 

Davis,  W.  M.     Die  Erklarende  Beschreibung  der  Landformen.     P.  473, 
Leipzig  and  Berlin,  1912. 

14.  Redman,  J.  B.     On  the  Alluvial  Formations,  and  the  Local   Changes 

of  the  South  Coast  of  England.      Min.  Proc.  Inst.  Civ.  Engr.      XI, 
162-204,  1852. 

15.  HowLETT,  B.  S.     [On  beach  ridges.]     Min.  Proc.  Inst.  Civ.  Eng.     XI, 

213,  1852. 

16.  CuBiTT,  Wm.     [On  sliingle  fulls.]     Min.  Proc.  Inst.  Civ.  Eng.     XI,  205, 

1852. 

17.  Cornish,  Vaughan.     On  Sea  Beaches  and  Sand  Banks.      Geog.  Jour. 

London.     XI,  538,  1898. 

18.  Wheeler,  W.  H.     The  Sea  Coast:    Destruction:    Littoral  Drift:    Pro- 

tection, p.  36,  London,  1902. 

19.  Solger,  F.     Dunenbuch,  pp.  51-52,  Stuttgart,  1910. 

20.  Keilhack,  K.     Die  Verlandung  der  Swinepforte.     Jahrbuch  der  Konigl. 

Preuss.  Geol.  LandesanstaJt  f.  1911.     XXXII,  231,  1912. 

21.  Johnson,  Douglas  W.  and  Reed,  W.  G.      The  Form  of  Nantasket 

Beach.     Jour,  of  Geol.     XVIII,  188,  1910. 

22.  Braun,     Gustav.     Einige  Ergebnisse    Entwickelungsgchichtlicher  Stu- 

dien  an  Europaischen  Flachlandskiisten  und  ihren  Dunen.     Zeits.  der 
Gesells.  flir  Erdkunde  zu  Berlin,  pp.  543-560,  1911. 

23.  Redman,  J.  B.     On  the  Alluvial  Formations,  and  the  Local  Changes 

of  the  South  Coast  of  England.      Min.  Proc.  Inst.  Civ.  Engr.      XI, 
174,  1852. 

24.  Ibid.,  p.  174. 

25.  Drew,  F.     [On  the  Dungeness]  quoted  by  Wm.  Topley  in  "The  Geology 

of  the  Weald."     Mem^  Geol.  Surv.  England  and  Wales.     P.  309,  1875. 

26.  Ibid.,  p.  214. 

27.  Ibid.,  p.  309. 

28.  Gulliver,  F.  P.     Dungeness  Foreland.     Geogr.  Jour.,  London.   IX,  539, 

1897. 

29.  Redman,  J.  B.     On  the  Alluvial  Formations,  and  the  Local   Changes 

of  the  South  Coast  of  England.      Min.  Proc.  Inst.  Civ.  Engr.     XI, 
173,  1852. 

30.  Gulliver,  F.  P.     Dungeness  Foreland.      Geog.  Jour.,  London.      IX,  539, 

1897. 

31.  Lewin,  Thomas.     The  Invasion  of  Britain  by  JuHus  Caesar.     2nd  Edi- 

tion, pp.  131  +  CXXIV,  London,  1862. 

32.  Burrows,  Montagu.     Cinque  Ports.     2nd  Edition,  p.  16,  London,  1888. 

33.  Drew,  F.     [On  the  Dungeness]  quoted  by  Wm.  Topley  in  "The  Geology 

of  the  Weald."     Mem.  Geol.  Surv.  England  and  Wales,  p.  308,  1875. 

34.  Gulliver,   F.   P.      Dungeness  Foreland.      Geog.   Jour.,   London.     IX, 

539,  1897. 

35.  Appach,  F.  H.     Caius  Julius  Caesar's  British  Expeditions  from  Boulogne 

to  the  Bay  of  Apuldore,  and  the  Subsequent  Formation,  Geologically 
of  Romney  Marsh.     143  pp.,  London,  1868. 

36.  Burrows,   Montagu.      Cinque  Ports.      2nd  Edition,  p.   16,   London, 

1888. 


REFERENCES  455 

37.  Robertson,  W.  A.  Scott.     The  Cinque  Port  Liberty  of  Romney.     Arch- 

seologica  Cantiana.     XIII,  261-280,  1880. 

38.  Braun,  Gustav.     Einige  Ergebnisse  Entwickelungsgeschichtlicher  Stu- 

dien  an  Europaischen  Flachlandsklisten  iind  ihren  Diinen.     Zeits.  der 
Gesells.  fiir  Erdkunde  zu  Berlin,  pp.  546-547,  1911. 
Braun,  Gustav.     Entwickelungsgeschichtliche  Studien  an  europaischen 
Flachlandsklisten  und  ihren  Diinen.     Veroff.  Inst    fiir  Meereskunde 
u.  s.  w.,  XV,  14-17,  Berhn,  1911. 

39.  Otto,  Theodor.      Der  Darss  und  Zingst.     Jahresb.  der  Geog.  Gesells. 

Greifswald.     XIII,  235-485,  1913. 

40.  Ihid.,  p.  330. 

41.  Ihid.,  p.  483. 

42.  IvEiLHACK,  K.     Die  Verlandung  der  Swinepforte.     Jahrbuch  der  Konigl. 

Preuss.  Geol.  Landesanstalt  fiir  1911.     XXXII,  232,  1912. 

43.  SoLGER,  F.     Dunenbuch,  pp.  46-65,  Stuttgart,  1910. 

44.  Keilhack,    K.       Die    Verlandung    der    Swinepforte.       Jahrbuch    der 

Konigl.  Preuss.  Geol.  Landesanstalt  fur  1911.  XXXII.  217-218.  225, 
1912. 

45.  Ibid.,  pp.  219,  223,  227. 

46.  Otto,  Theodor.      Der  Darss  und  Zingst.      Jahresb.  der  Geog.  Gesells. 

Greifswald.    XIII,  337,  1913. 

47.  Ibid.,  p.  484. 

48.  Iveilhack,  K.     Die  Verlandung  der  Swinepforte.     Jahrbuch  der  Konigl. 

Preuss.  Geol.  Landesanstalt  fur  1911.     XXXII,  221,  1912. 

49.  Ihid.,  pp.  224-225. 

50.  Ihid.,  p.  231. 

51.  Ibid.,  p.  225. 

52.  Kruger,   Gustav.     Uber  Sturmfluten  an  der  Deutschen  Kusten  der 

Westhchen  Ostsee  mit  Besonderer  Beriicksichtigung  der  Sturniflut 
vom  30-31  Dezember,  1904.  Jahresb.  der  Geogr.  Gesells.  zu  Greifs- 
wald, 1909-1910.     XII,  220-223,  1911. 

53.  Keilhack,  K.     Die  Verlandung  der  Swinepforte.     Jahrbuch  der  Konigl. 

Preuss.  Geol.  Landesanstalt  fiir  1911.     XXXII,  227,  1912. 

54.  Ihid.,  p.  217. 

55.  Wkeeler,  W.    H.     The  Sea  Coast:  Destruction:    Littoral  Drift:    Pro- 

tection, pp.  39,  144,  London,  1902. 

56.  GoLDTHWAiT,  J.  W.     Supposed   Evidences   of  Subsidence  of  the  Coast 

of  New  Brunswick  within  Modern  Times.  Can.  Geol.  Surv.,  Museum 
Bulletin  No.  2,  p.  21  (of  reprmt)  1914. 

57.  Keilhack,  K.     Die  Verlandung  der  Swinepforte.     Jahrbuch  der  Konigl. 

Preuss.  Geol.  Landesanstalt  fiir  1911.     XXXII,  225,  1912. 

58.  SoKOLOw,  N.  A.     Die  Diinen:   Bildung,  Entwickelung  und  Innerer  Bau, 

p.  39,  Berhn,  1894. 
Beaurain,  G.     Quelques  Faits  Relatifs  a  la  Formation  du  Littoral  des 
Landes  de  Gascogne  Revue  de  Geog.     XXVIII,  255,  1891. 

59.  Goldthwait,  J.  W.     Supposed  Evidences  of  Subsidence  of  the  Coast 

of  New  Brunswick  within  Modern  Time.  Can.  Geol.  Surv.^  Museum 
Bulletin,  No.  2,  p.  20  (of  reprint),  1914. 


456  SHORE  RIDGES   AND  THEIR  SIGNIFICANCE 

60    Cornish,  Vatjghan.     On  Sea  Beaches  and  Sand  Banks.     Geog.  Jour, 

London.     XI,  538,  1898.  .  .v,    n      .    f 

fit     GoLDTHWAiT,  J.  W.     Supposed  Evidences  of  Subsidence  of  the  Coast  ol 

New  Brunswick  within  Modern  Time.     Can.  Geol.  Surv.,  Museum 

BuUetin  No.  2,  pp.  1-23  (of  reprint),  1914. 
62    Johnson,  Douglas  W.  and  Reed,  W.  G.       The  Form  of  Nantasket 

Beach.    Jour.ofGeoL     XVIII,  162-189,  1910. 


CHAPTER  X 
MINOR  SHORE  FORMS 

Advance  Summary.  —  There  remain  for  consideration  a  number 
of  shore  forms  which  are  not  of  primary  significance  in  a  discus- 
sion of  shoreline  development,  but  which  are  nevertheless  of 
much  importance  to  the  geographer  and  geologist,  and  in  some 
cases  also  to  the  engineer.  It  is  proposed  to  give  some  account 
of  these  features  in  the  present  chapter.  Beach  cusps  are  first 
discussed  at  much  length,  after  which  the  low  and  ball,  especially 
characteristic  of  sandy  shores,  are  described.  Ripple  marks  re- 
ceive an  extended  treatment,  following  which  rill  marks,  swash 
marks,  backwash  marks,  sand  domes,  and  shore  dunes  each  in 
turn  are  briefly  considered. 

Beach  Cusps.  —  Among  the  minor  forms  of  the  shore  zone 
none  has  proved  more  puzzling  than  the  cuspate  deposits  of 
beach  material  built  by  wave  action  along  the  foreshore.  Sand, 
gravel,  or  coarse  cobblestones  are  heaped  together  in  rather 
uniformly  spaced  ridges  which  trend  at  right  angles  to  the  sea 
margin,  tapering  out  to  a  point  near  the  water's  edge.  These 
"  beach  cusps  "  have  attracted  the  attention  of  many  students, 
and  it  will  be  profitable  for  us  to  consider  first  the  opinions  of 
other  writers  concerning  them;  then  of  examine  more  carefully 
into  their  essential  characteristics;  and  finally  to  criticize  the 
various  theories  which  have  been  proposed  to  account  for  their 
origin  and  development. 

Previous  Studies  of  Beach  Cusps.  —  The  earliest  account  of 
beach  cusps  which  has  come  to  my  attention  occurs  in  a  paper 
on  shingle  beaches  published  by  Palmer^  in  1834.  Palmer's 
description  of  the  forms  is  very  vague,  but  he  recognized  the 
important  fact,  not  appreciated  by  all  later  students,  that  the 
cusps  are  produced  by  waves  "  driven  directly  upon  the  beach," 
whereas  they  are  destroyed  when  "  an  oblique  direction  is  given 
to  the  motion  of  the  waves."  In  an  unpublished  thesis,  "  The 
Geology  of   Nahant  "  written   by  Lane   about   1887,  the  cusps 

457 


458 


MINOR  SHORE   FORMS 


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BEACH  CUSPS  459 

on  Lynn  Beach,  Massachusetts,  are  briefly  described  and  their 
origin  discussed.  Lane  concluded  that  cusps  are  formed  by 
the  action  of  waves  parallel  to  the  coast;  that  they  have  their 
beginnings  in  accidental  irregularities  on  the  beach;  that  they 
become  evenly  spaced  as  the  result  of  some  process  of  adjust- 
ment not  clearly  understood,  and  that  the  distance  between 
cusps  is  in  some  manner  related  to  the  height  of  the  waves  and 
the  breadth  of  the  beach.  A  short  abstract  of  this  thesis  was  pub- 
lished in  1888,  but  contains  only  a  brief  reference  to  the  cusps^. 

A  few  years  later  Shaler,  in  his  popular  treatise,  "  Sea  and 
Land^,"  gave  a  clear  description  of  the  curious  "  ridges  and 
furrows  "  occurring  on  shores,  recognized  their  temporary  char- 
acter and  the  ease  with  which  they  are  obliterated  by  wave 
action,  and  expressed  the  opinion  that  "  the  origin  of  these 
peculiar  structures  is  not  easily  accounted  for."  Shaler  pub- 
lished a  somewhat  fuller  account  of  beach  cusps  in  his  paper  on 
"  Beaches  and  Tidal  Marshes  of  the  Atlantic  Coast."  A  theory 
of  origin  was  there  proposed  in  the  following  words: 

"  It  seems  to  the  writer  that  these  scallops  were  formed  about 
as  follows:  In  a  time  of  storm  the  inner  edge  of  the  swash  Ime 
formed  by  the  body  of  water  which  sweeps  up  and  down  the 
beach  has  a  very  indented  front,  due  to  the  fact  that  it  is  shaped 
by  a  criss-cross  action  of  many  waves.  As  these  tongues  run 
up  the  beach  and  strike  the  pebbles,  they  push  them  back  so 
as  to  make  a  slight  indentation  where  each  tongue  strikes.  As 
the  water  goes  back,  it  pulls  out  the  fine  material,  but  does  not 
withdraw  the  pebbles.  The  next  stroke  of  the  splashing  water 
then  finds  a  small  bay,  the  converging  horns  of  which  slightly 
heap  up  the  fluid,  making  the  stroke  a  little  harder  in  the  center 
of  the  tongue  and  excavating  the  bottom  of  the  bay  still  farther. 
As  the  re-entrant  grows  larger  and  the  tide  rises  higher,  the  water, 
as  it  runs  up,  forms  a  small  wave,  which  breaks  on  the  shore  of 
the  recess  and  casts  the  pebbles  more  into  the  form  of  a  ridge. 
This  action,  continuing  for  some  hours  before  the  tide  turns, 
serves  to  shape  the  embayment. 

"  It  should  be  carefully  noted  that,  when  the  swaying  waters 
rush  up  into  the  shore  scallops,  the  converging  walls  of  these 
indentations  deepen  the  current  and  add  to  the  efficiency  of  its 
movements  —  a  process  which  is  essentially  like  that  which  is 
brought  about  when  an  ordinary  wave  enters  into  a  recess  of  the 


460  MINOR  SHORE  FORMS 

cliff,  or  the  tidal  undulation  is  crowded  into  an  indentation 
such  as  the  Bay  of  Fundy^." 

In  his  paper  on  "  Sea-beaches  and  Sand-banks  "  published  in 
1898,  Cornish  briefly  refers  to  the  "  succession  of  ridge  and 
furrow  at  right  angles  to  the  sea-front,"  and  attributes  the 
phenomenon  to  the  erosive  action  of  waves  which  are  increasing 
in  size  and  attempting  to  reduce  the  beach  slope  to  a  gentler 
gradient.  A  variation  of  the  same  feature  is  described  by 
Cornish  under  the  name  "Shingle  Barchanes."  He  was  of  the 
opinion  that  the  shingle  barchanes  were  analogous  to  that  form 
of  sand  dune  called  a  barchane,  and  considered  any  discussion 
of  theh  origin  superfluous^ 

One  year  later  Jefferson  published  a  paper  in  which  he  de- 
scribed some  of  the  characteristic  features  of  beach  cusps  and 
offered  an  explanation  of  their  origin.  Jefferson's  studies  were 
"  made  at  a  single  beach  (Lynn  Beach,  Massachusetts),  though 
confirmed  by  some  observations  from  Gay  Head  and  Narragan- 
sett  Bay."  He  concluded  that  the  cusps  were  caused  by  the 
escape  of  water  from  behind  a  barrier  of  seaweed  located  near 
the  upper  zone  of  the  beach.  Occasional  waves  of  more  than 
average  size  overtop  the  seaweed  barrier  and  leave  large  quan- 
tities of  water  imprisoned  behind  it.  After  the  retreat  of  the 
wave  the  imprisoned  water  escapes  through  occasional  breaches 
in  the  barrier  and  flows  down  the  beach  in  streams  of  consider- 
able strength,  which  scour  away  the  beach  material  along  their 
courses.  The  residual  masses  of  material  thus  left  between  the 
stream  lines  are  gradually  shaped  by  the  waves  into  typical 
beach  cusps.  A  stony  barrier  would  probably  not  operate  in 
the  same  manner  as  a  barrier  of  seaweed,  since  the  water  would 
filter  through  the  mass  rather  than  wear  channels.  "  It  would 
seem  to  follow  that  such  stony  cusps  are  to  be  looked  for  only 
on  coasts  where  seaweed  or  some  similar  material  is  abundantly 
thrown  up^" 

In  1900  Branner  published  a  paper  entitled  "  The  Origin  of 
Beach  Cusps,"  based  on  observations  made  on  the  California 
coast  and  the  northeast  coast  of  Brazil.  He  noted  the  fact  that 
cusps  occur  where  "  there  are  no  seaweeds  or  other  '  drift '  on 
the  beach,"  and  concluded  that  they  are  formed  "  by  the  inter- 
ference of  two  sets  of  waves  of  translation  upon  the  beach." 
The  accompanying  diagrams,  reproduced  from  Branner's  paper, 


BEACH  CUSPS 


461 


will  serve  to  make  his  theory  clear.  In  Figure  139  "the  concen- 
tric lines  represent  two  sets  of  waves  advancing  on  the  beach  in 
the  direction  indicated  by  the  arrows  and  crossing  each  other  along 
the  broken  lines.  In  deep  water  these  are  waves  of  oscillation, 
but  when  they  reach  the  shallow  water  on  the  beach  they  become 
waves  of  translation  and  interfere  with  each  other  where  they 
converge  upon  the  shore.     The  tendency  is  for  them  to  check 


Fib.  139.  —  Diagram  illustrating  Branner's  theory  of  beach  ciisp  formation. 


Fig.  140.  —  Diagram  illustrating  Branner's  theory  of  the  formation  of  un- 
equally spaced  beach  cusps.  If  DC  were  the  beach,  the  cusps  would  be 
uniformly  spaced. 

each  other  along  these  lines  of  interference  and  to  heap  up  the 
sands  at  the  points  marked  A,  where  they  strike  the  beach. 
At  the  points  marked  B  the  waves  diverge  and  throw  the  beach 
sands  and  all  floating  material  alternately  right  and  left." 

"  In  Figure  140  the  waves  are  represented  as  breaking  on  a 
straight  beach.  If  the  water  offshore  were  of  a  uniform  depth 
and  the  waves  were  evenly  spaced,  the  cusps  in  this  case  would, 
for  obvious  reasons,  be  farther  and  farther  apart  from   left  to 


462  MINOR  SHORE  FORMS 

right,  as  shown  along  the  beach  DE.  The  distance  between 
the  cusps  is  equal  to  the  spaces  measured  on  the  beach  between 
the  radii  along  which  the  wave  interference  approaches  the 
shored"  In  an  editorial  note  in  the  Journal  of  Geology  for 
1901,  Branner  briefly  restated  his  theory  of  cusp  formation,  and 
called  attention  to  the  fact  that  "  giant  ripples  "  and  similar 
beach  structures  observed  in  sedimentary  rocks  may  be  fossil 
beach  cusps^. 

Among  the  "  Author's  abstracts  of  papers  read  at  the  Wash- 
ington meeting  of  the  American  Association  for  the  Advance- 
ment of  Science,  Section  E,"  published  in  the  Journal  of  Geology 
for  1903,  is  an  abstract  of  a  paper  by  Jefferson  entitled  "  Shore 
Phenomena  on  Lake  Huron."  The  abstract  suggests  a  modi- 
fication of  the  author's  views  as  published  four  years  before; 
for  while  in  the  earlier  paper  the  possibility  of  a  stony  barrier's 
playing  the  same  part  in  cusp  formation  as  a  seaweed  barrier  is 
considered  and  rejected  as  improbable,  in  the  later  paper  we 
read  that  the  cusps  are  "  component  features  of  a  beach  ridge,  .  .  . 
The  ridge  .  .  .  has  at  times  been  seen  and  photographed  with 
water  caught  behind  and  rushing  out  at  breaks  in  the  line,  as 
with  the  weed  line  at  Lynn^."  Whether  or  not  the  breaking 
of  water  through  the  barrier  is  still  thought  to  orighiate  the 
cusps  is  not  made  clear.  The  cross-waves  noted  by  Branner 
were  observed  by  Jefferson,  but  at  no  place  did  he  find  such 
waves  associated  with  cusp  formation. 

Alexander  Agassiz  in  a  report  on  "  The  Coral  Reefs  of  the 
Tropical  Pacific^*^,"  figures  a  series  of  "  boulder  cusps  "  observed 
on  the  shores  of  Arhno  atoll.  Judging  from  the  illustration 
these  are  true  beach  cusps;  but  the  method  of  origin  advocated 
by  Agassiz  is  that  described  on  an  earlier  page  of  the  present 
volume  for  the  formation  of  cobblestone  deltas  in  marshes  or 
lagoons  by  waves  washing  over  a  low  beach.  The  position  of 
the  "  boulder  cusps  "  on  the  shores  of  a  narrow  lagoon,  is  com- 
patible with  the  delta  theory  rather  than  with  the  beach  cusp 
theory;  but  the  forms  as  figured  could  not  have  been  produced 
by  over  washing  waves.  Some  doubt  must  therefore  attach  to 
Agassiz's  brief  observations. 

In  his  paper  "  Cuspate  Forelands  along  the  Bay  of  Quinte  "^^ 
A.  W.  G.  Wilson  describes  the  occurrence  of  "  cusplets  "  on 
one  of  the  forelands,  and  ascribes  them  to  the  action  of  a  single 


BEACH  CUSPS  463 

series  of  waves  striking  the  beach  at  an  obUque  angle.  Although 
Wilson  does  not  refer  to  the  previously  pubhshed  accounts,  and 
although  the  very  asymmetrical  forms  described  by  him  differ 
in  some  respects  from  the  essentially  symmetrical  features  gen- 
erally known  as  beach  cusps,  there  is  little  reason  to  doubt 
that  the  former  are  modified  phases  of  the  latter. 

In  1905  Jefferson  published  a  paper  entitled  "  On  the  Lake 
Shore  "^2,  in  which  he  gives  a  brief  account  of  beach  cusps, 
and  says  "  they  never  occur  except  after  waves  that  have  played 
squarely  on  shore."  Examples  which  must  have  formed  with- 
out the  aid  of  seaweed  barrier  are  figured,  but  their  origin  is 
not  explained.  In  referring  to  one  particular  set,  however, 
Jefferson  classes  them  with  the  Lynn  beach  cusps,  and  says: 
"  Some  high  wave  surmounts  the  ridge,  here  of  sand,  there  of 
seaweed,  and  its  crest  water  is  ponded  behind  it  to  escape  by  any 
sags  that  may  occur  in  the  line." 

My  own  attention  was  first  directed  to  the  study  of  beach 
cusps  in  the  fall  of  1903.  Seven  years  later  I  discussed  their 
form  and  origin  in  a  paper  published  in  the  Bulletin  of  the  Geo- 
logical Society  of  America'^,  and  it  is  upon  this  paper  that  the 
present  discussion  of  beach  cusps  is  largely  based. 

Characteristics  of  Beach  Cusps.  —  When  most  perfectly  de- 
veloped, the  ideal  beach  cusp  has  a  shape  suggesting  an  isosceles 
triangle,  and  is  so  placed  that  the  unequal  side  (hereafter  called 
the  base)  is  parallel  to,  but  farthest  from,  the  shoreline.  The 
"  triangle  "  may  be  short  and  blunt,  or  may  be  so  greatly  elon- 
gated that  the  two  equal  sides  extend  far  down  the  beach  and 
finally  unite  to  form  an  acute  point  (hereafter  called  the  apex). 
These  same  sides  may  be  relatively  straight,  ])ut  are  more  often 
concave,  sometimes  convex,  outward.  The  actual  variations  in 
form  are  numerous  and  wide  (Fig.  141).  Every  gradation  can 
be  found  from  well  developed  triangular  accumulations  of  sand 
or  gravel  to  widely  spaced  heaps  of  cobblestones  of  no  definite 
shape.  The  cusps  may  constitute  the  serrate  seaward  side  of  a 
prominent  beach  ridge,  or  may  occur  as  isolated  gravel  hillocks 
separated  by  fairly  uniform  spaces  of  smooth  sandy  beach. 
They  may  be  sharply  differentiated  from  the  rest  of  the  beach, 
or  may  occur  as  gentle  undulations  of  the  same  material  as  the 
beach  proper,  and  so  be  scarcely  discernil)le  as  independent 
features.     Indeed,  the  variations  in  beach  cusps  are  so  great 


464 


MINOR  SHORE  FORMS 


h3 


BEACH  CUSPS  465 

that  their  form  is  often  not  as  sure  a  guide  to  their  detection  as 
is  their  systematic  recurrence  at  fairly  uniform  intervals.  One 
or  two  indefinite  heaps  of  gravel  on  a  beach  would  escape  notice, 
but  a  hundred  such  heaps,  evenly  spaced,  attract  attention. 

A  cusp  may  rise  from  an  inch  or  less  to  several  feet  above 
the  general  level  of  the  beach.     Many  are  relatively  low  and 


ifti^^^^WiP^'ip: 


Fig.  141.  — Variations  in  the  form  of  beach  cusps. 

flat,  others  high  and  steep-sided.  Sometimes  the  highest  part 
is  comparatively  near  the  apex;  at  other  times  the  highest 
part  is  far  back,  and  from  it  a  long,  sloping  ridge  trails  forward 
toward  the  water.  As  a  rule,  the  cusps  appear  to  point  straight 
out  toward  the  water,  and  neither  side  of  a  cusp  is  steeper  than 
the  other  except  where  oblique,  wind-made  waves  have  eroded 
one  side  only,  a  condition  observed  in  a  few  cases. 

An  interesting  variation  in  form  is  found  where  old  cusps  ter- 
minate abruptly  in  little  "  cliffs  "  instead  of  in  sharp  points 
(Plate  LVI).  It  is  plain  that  after  the  old  cusps  had  been 
formed  they  were  cliffed  by  waves  under  changed  conditions  and 
their  apices  cut  away.  From  this  eroded  material  later  series  of 
cusps  may  form,  unrelated  in  position  to  the  original  series.  Fig- 
ure 142  represents  a  case  of  this  kind  as  observed  in  cobblestone 


466  MINOR  SHORE  FORMS 

and  gravel  cusps  on  a  gravel  beach  at  Winthrop,  Massachusetts. 
Sometimes  the  cusps  are  more  completely  eroded  than  in  the 
case  figured,  and  remnants  of  three  or  four  distinct  sets,  of  differ- 
ent sizes  and  spacing,  may  often  be  observed  on  a  beach  at  one 
time. 

As  in  the  form  of  cusps,  so  in  the  material  of  which  they  are 


Fig.  142.  —  Partially  eroded  older  cusps  and  respaced  later  series. 

composed,  is  there  the  widest  variation.  In  building  them  the 
waves  make  use  of  everything,  from  the  finest  sand  to  the 
coarsest  col^blestones.  There  is  no  necessary  relation  between 
the  size  of  the  cusp  and  the  size  of  the  material  of  which  it  is 
composed.  Large  cusps  built  wholly  of  fine  sand  are  reported 
from  Virginia  Beach,  and  still  larger  ones  (20  to  30  feet  from 
apex  to  base  and  75  to  90  feet  between  apices)  built  of  similar 
material  were  observed  on  the  beach  south  of  Dyker  Heights 
on  Long  Island.  Kemp^^  has  studied  large  sand  cusps  on  Mel- 
bourne Beach,  Florida,  which  measured  from  90  to  95  feet 
between  apices  and  rose  at  least  3  or  4  feet  above  the  general 
level  of  the  beach.  The  largest  examples  are  more  often  built 
of  coarse  gravel  or  cobblestones,  while  small  ones  may  be  com- 
posed of  either  fine  sand  or  coarse  gravel.  The  very  smallest 
cusps,  measuring  a  few  inches  in  length,  consist  of  fine  material 
only,  since  the  small  waves  which  build  them  cannot  transport 
coarse  gravel  or  cobblestones.  Where  both  coarse  and  fine 
materials  occur  on  a  beach,  the  cusps  are  built  of  the  coarse 
material.  Gravel  cusps  on  a  sandy  beach  are  of  common  occur- 
ence, but  I  have  not  observed  sand  cusps  on  a  gravel  beach. 
The  smallest  cusps  which  have  come  under  ni}^  observation 


BEACH  CUSPS  467 

have  been  those  artificially  produced  in  the  laboratory.  These 
have  varied  from  an  inch  to  several  inches  in  length,  measured 
from  apex  to  base.  Some  almost  as  small  are  to  be  found  along 
the  shores  of  sheltered  ponds.  On  a  sandy  beach  at  the  head 
of  a  protected  bay  south  of  Huletts  Landing,  Lake  George, 
cusps  from  8  to  12  inches  long  were  formed  by  the  small  waves 
set  in  motion  by  a  gentle  breeze.  Those  found  along  the  sea- 
shore may  reach  a  length  of  30  feet  or  more.  It  should  be 
noted,  however,  that  the  length  measured  from  apex  to  base  is 
less  significant  than  the  distance  between  cusps,  measured  from 
apex  to  apex;  for  while  it  is  a  general  rule  that  the  farther 
apart  the  cusps  the  larger  is  their  size,  some  which  are  closely 
spaced  may  be  greatly  elongated,  as  pointed  out  above,  and  this 
elongation  appears  to  be  the  result  of  rather  accidental  condi- 
tions, and  to  have  no  great  significance.  Measurements  across 
the  bases  might  be  more  significant,  but  it  is  often  difficult  to 
determine  the  length  of  base,  as  when  the  cusps  form  part  of 
a  beach  ridge  or  constitute  widely  separated  heaps  of  gravel 
having  a  vague  shoreward  boundary.  However,  enough  has 
been  said  to  give  some  idea  of  the  range  in  size;  and  although 
size  is  in  some  degree  related  to  spacing,  the  latter  is  the  really 
important  factor,  as  noted  below. 

The  very  small  cusps  made  in  the  laboratory  are  from  one  to 
several  inches  apart,  measured  from  apex  to  apex.  On  the  shore 
of  small  ponds  and  bays,  where  only  small  waves  are  developed, 
the  spacing  varies  from  less  than  a  foot  to  two  feet  or  more. 
On  sea  beaches  the  cusps  built  by  small  waves  may  be  less  than 
10  feet  apart,  while  those  built  by  large  storm  waves  may  be 
100  fee+  apart. 

Jefferson  emphasizes  the  lack  of  regularity  in  the  spacing  of 
cusps,  whereas  others  have  been  impressed  by  their  regular 
recurrence  at  fairly  uniform  intervals.  Liasmuch  as  the  matter 
of  spacing  is  of  vital  importance  in  any  discussion  of  the  origin 
of  these  forms,  we  may  examine  it  somewhat  carefully.  Jeffer- 
son^^ writes:  "  The  constant  recurrence  of  bay  (intercusp  space) 
and  point  (apex)  as  one  walks  along  the  beach  suggests  that 
there  is  a  regularity  in  the  width  of  intervals.  This  is  not  so, 
however,  on  Lynn  Beach,  as  appears  from  the  diagram,  meas- 
ures from  point  to  point  along  the  beach  being  21,  20,  18,  16,  22. 
17,   6,   7,   and  22  paces.     Fainter  cusps  farther  south  toward 


468 


MINOR  SHORE  FORMS 


> 


a 


BEACH  CUSPS  469 

Nahant  show  similar  irregulaiity.  It  might  be  said,  however 
that  on  Lynn  Beach  they  are  commonly  about  20  paces  wide." 
And  again^'':  "  In  a  view  along  the  beach  these  unevennesses 
are  foreshortened  into  the  appearance  of  points  of  sand  or  gravel 
known  as  beach  cusps.  They  are  less  even  than  they  look." 
In  still  another  connection  he  says:  "Perspective  foreshortening 
gives  them  a  fictitious  appearance  of  regularity"."  On  the 
other  hand,  Shaler^^  speaks  of  their  "  orderly  and  uniform  suc- 
cession ";  and  it  has  seemed  to  me  that  the  degree  of  regu- 
larity in  spacing  is  so  great  as  to  be  incompatible  with  certain 
of  the  proposed  theories  of  origin. 

It  is  true  that  measurements  of  the  spaces  do  not  always  give 
exactly  the  same  figure;  that  in  the  early  stages  of  development 
a  greater  degree  of  irregularity  prevails  than  later  on ;  and  that 
even  where  cusps  are  very  perfectly  developed,  occasional  aber- 
rant features  obscure  the  regularity  of  spacing.  Nevertheless, 
a  large  number  of  observations  of  beach  cusps  in  all  stages  of 
formation  and  destruction,  and  the  production  of  artificial  cusps 
in  the  laboratory  have  convinced  me  that  a  fairly  high  degree  of 
regularity  in  spacing  is  a  most  characteristic  feature  of  well 
developed  forms  and  must  carefully  be  considered  in  any  attempt 
to  account  for  their  origin. 

The  width  of  the  intercusp  spaces  varies  with  the  size  of  the 
waves.  When  the  waves  are  about  an  inch  in  height  the  cusps 
are  from  3  to  9  inches  apart;  when  the  waves  are  from  one  and 
a  half  to  two  and  a  half  feet  high  they  are  30  to  60  feet  apart, 
while  large  storm  waves  build  cusps  100  feet  or  more  apart. 
These  figures  are  only  approximate,  and  are  based  on  rough 
estimates  of  the  wave  height  close  to  the  shoreline.  Sufficient 
data  have  not  been  secured  on  which  to  base  a  reliable  deter- 
mination of  the  precise  relation  of  intercusp  space  to  wave 
height,  but  within  certain  limits  there  is  a  suggestion  that 
doubling  the  v/ave  height  doubles  the  length  of  the  space.  A 
large  number  of  careful  observations  would  probably  establish 
this  point.  In  conducting  such  an  investigation  the  observer 
must  satisfy  himself  that  the  waves  he  sees  are  actually  building 
the  cusps,  for  waves  of  any  size  may  play  about  cusps  formed 
by  other  waves  of  different  size,  and  thus  mislead  one  who 
compares  the  intercusp  spaces  with  the  height  of  the  later 
waves.     Fortunately  a  given  set  of  waves  does  not  long  leave 


470  MINOR  SHORE  FORMS 

unmolested  a  series  of  cusps  formed  by  waves  of  an  entirely- 
different  size,  and  the  patient  observer  can  in  time  determine 
whether  or  not  the  waves  then  breaking  on  the  beach  are  to  be 
correlated  with  the  cusps  at  the  water's  edge. 

This  brings  us  to  the  consideration  of  another  significant 
point  in  connection  with  the  spacing  of  beach  cusps:  namely, 
the  relative  ease  with  which  old  cusps  are  remodeled  by  waves 
differing  in  size  from  those  which  formed  them.  If  closely 
spaced  cusps  formed  by  small  waves  are  attacked  by  larger 
waves,  there  ensues  a  rearrangement  by  which  the  cusps  become 
larger  and  farther  apart.  This  rearrangement  may  be  gradual, 
and  may  be  accompanied  by  the  combining  of  some  cusps  and 
the  slow  obliteration  of  others;  or  if  the  new  waves  are  very 
large,  there  may  be  a  rapid  obliteration  of  the  earlier  series  of 
cusps,  followed  by  the  slow  formation  of  a  new  series  adjusted 
to  the  size  of  the  later  waves.  If  the  widely  spaced  cusps  formed 
by  large  waves  are  attacked  by  smaller  waves,  so  much  of  the 
older  cusps  as  can  be  reached  will  be  eroded  and  the  material 
refashioned  into  smaller  cusps  more  closely  spaced,  regardless 
of  the  positions  of  the  older  ones  (Fig.  142).  When  large  and 
.widely  spaced  cusps  are  built  by  high  storm  waves  well  up  the 
slope  of  the  beach,  only  their  apices  are  apt  to  be  attacked  by 
the  smaller  waves  of  calmer  weather,  and  so  it  happens  that  we 
commonly  find  the  largest  cusps  partially  preserved  near  the 
top  of  the  beach,  with  series  of  smaller  and  more  closely  spaced 
cusps  farther  down  the  slope. 

Regarding  the  building  of  beach  cusps,  Jefferson^^  writes: 
"  If  it  be  asked  how  this  begins,  the  answer  must  be  that  the 
beginning  is  as  old  as  the  beach.  .  .  .  Each  set  of  cusps 
may  modify  its  successors.  A  new  crest  of  seaweed  flung  up 
today  is  likely  to  have  its  weak  points  in  some  measure  deter- 
mined by  the  previous  channels.  In  violent  storms  it  is  doubtful 
if  this  control  is  significant.  Each  storm  probably  sets  the 
shape  in  which  the  waves  must  play  for  a  long  time."  If  we 
accept  Jefferson's  theory  of  cusp  formation,  the  conclusions  just 
quoted  would  seem  to  be  reasonable.  But  the  sensitiveness  of 
beach  cusps  to  changes  in  size  of  waves  leads  to  quite  opposite 
conclusions.  Instead  of  the  beginning  of  cusp  formation  dating 
back  indefinitely,  there  appears  to  be  a  new  and  quite  inde- 
pendent beginning  with  every  marked  change  in  the  size  of 


/ 
BEACH  CUSPS  471 

waves.  One  set  of  cusps  seems  to  have  little  influence  on  the 
position  of  its  successors.  Along  the  shores  of  a  little  bay  just 
south  of  Huletts  Landing,  Lake  George,  cusps  built  by  small 
waves  are  completely  obliterated  each  day  by  three  or  four  of 
the  large  waves  which  strike  the  beach  after  the  passing  of  a 
steamboat.  Opposite  the  cusps,  but  farther  up  the  beach,  pegs 
were  driven  to  mark  the  position  of  the  cusps.  After  their 
obliteration  they  formed  again  under  the  influence  of  the  small 
waves,  with  the  same  size  and  spacing  as  before,  but,  as  shown 
by  the  pegs,  in  totally  new  positions.  The  law  controlling  the 
relation  of  spacing  to  wave  size  was  operative,  but  the  cusps 
which  were  there  a  few  moments  before  did  not  determine  the 
position  of  their  successors.  The  same  phenomenon  may  be 
observed  in  the  production  of  artificial  cusps.  Furthermore,  if 
a  series  of  parallel  trenches  be  excavated  in  the  artificial  beach 
at  right  angles  to  the  shoreline,  the  intercusp  spaces  and  the 
cusps  will  not  correspond  with  the  trenches  and  intervening 
ridges  which  have  been  made  to  guide  wave  action.  In  fact, 
waves  of  a  given  size  insist  on  forming  cusps  at  appropriate 
intervals,  and  while  their  action  may  be  influenced  within  cer- 
tain limits  by  natural  or  artificial  trenches  on  the  beach,  they 
refuse  to  be  controlled  by  such  depressions  unless  these  are 
themselves  appropriately  spaced.  Kemp^"  reports  that  at  Mel- 
bourne Beach  on  the  Florida  coast  continuous  observations 
throughout  one  winter  show  that  the  cusps  of  one  day  may  be 
completely  obliterated  in  a  few  hours,  and  the  beach  left  feature- 
less and  smooth.  The  next  series  of  waves  will  form  a  new  series 
of  cusps  quite  unrelated  in  spacing  to  the  earlier  series. 

The  bases  of  the  cusps  often  merge  with  the  last  formed 
beach  ridge  in  such  a  manner  as  to  leave  no  doubt  that  they 
constitute  an  integral  part  of  it.  The  ridge  may  or  may  not  be 
breached  opposite  the  intercusp  spaces;  but  it  should  be  noted 
that  with  the  progressive  concentration  of  the  water  in  the 
intercusp  spaces,  which  converge  shoreward,  the  parts  of  the 
ridge  most  likely  to  be  broken  through  are  the  parts  opposite 
these  spaces.  It  is,  therefore,  not  necessary  to  regard  the  in- 
tercusp spaces  as  the  product  of  erosion  by  water  which  was 
imprisoned  back  of  the  ridge  and  broke  through  it,  either  at 
the  lowest  places  or  at  points  of  weakness.  Conclusive  evidence 
that  the  ridge  may  be  breached  from  the  seaward  side  is  found 


472 


MINOR  SHORE  FORMS 


> 


■  '  ^  ♦. 


m 


BEACH  CUSPS  473 

in  the  gravel  or  cobblestone  deltas  which  are  sometimes  built 
landward  from  the  gap  in  a  ridge  at  the  head  of  an  intercuop 
space  (Fig.  143).  It  seems  clear  that  the  water  concentrated 
between  cusps  broke  through  the  rklge  and  carried  gravel  and 
cobbles  into  the  area  back  of  it.  In  one  case  observed  at  Nahant 
the  landward  projection  of  cobblestone  accumulations  was  so 
systematic  as  to  give  a  series  of  "  inverted  cusps  "  alternating 


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—         —     -;—   " 

Fig.  143.  —  Normal  and  inverted  beach  casps. 

regularly  with  the  beach  cusps  proper.  The  breaching  of  the 
ridge  by  water  concentrated  between  previously  formed  cusps 
has  been  repeatedly  observed  in  the  laboratory  experiments. 

There  are  abundant  instances  of  cusps  unrelated  to  any  beach 
ridge.  Cusps  of  gravel  are  often  formed  at  widely  separated 
intervals  with  smooth,  sandy  beach  between;  the  points  of  old 
cusps  are  nipped  ofi  and  respaced  without  the  development  of  a 
ridge.  One  must  conclude  that  cusps  may  develop  as  the  ser- 
rate seaward  margin  of  a  beach  ridge  and  may  determine  the 
places  where  it  will  be  Ijreached  by  the  waves,  but  that  there  is 
no  necessary  relation  between  the  two. 

The  return  current  of  water  flowing  down  the  beach  after 
the  wave  has  ended  its  advance,  sweeps  seaward  more  or  less 
fine  material  which  is  deposited  to  form  the  shoreface  terrace. 
When  cusps  have  not  formed,  the  margin  of  this  terrace  is  rela- 
tively straight;    but  after  cusps  have  developed,  the  greatest 


474  MINOR  SHORE  FORMS 

amount  of  water  and  debris  returns  down  the  slope  from  the 
intercusp  spaces,  building  the  subaqueous  platform  seaward 
more  rapidly  than  does  the  smaller  amount  of  water  and  debris 
returning  from  around  the  apices  of  the  cusps.  In  this  way  the 
margin  of  the  platform  becomes  scalloped,  each  intercusp  space 
having  a  scallop  or  miniature  delta  to  correspond  with  it.  It  is 
evident  that  the  scalloping  of  the  platform  presents  no  difficulty 
if  the  origin  of  the  cusps  is  understood. 

E elation  of  Beach  Cusps  to  other  Factors  of  Shore  Activity.  — 
In  collecting  data  concerning  beach  cusps  some  attention  has 
been  given  to  several  other  factors  of  shore  activity,  in  view 
of  the  possibility  that  they  might  exert  some  influence  on 
cusp  formation.  Several  of  these  factors  are  briefly  treated 
below. 

It  was  thought  at  first  that  the  angle  of  beach  slope  might 
exert  an  important  control  over  the  spacing  of  the  cusps,  inas- 
much as  the  slope  aflfects  both  the  volume  and  velocity  of  the 
water  advancing  and  retreating  over  the  zone  of  wave  attack. 
It  soon  became  apparent,  however,  that  if  the  inclination  of  the 
beach  does  influence  the  spacing,  the  effect  is  largely  masked  by 
the  far  more  important  factor  of  wave  size.  I  still  think  it 
probable  that  the  slope  of  the  beach  plays  a  small  part  in  the 
spacing  of  cusps,  but  have  not  sufiicient  data  on  this  point  to 
demonstrate  the  truth  of  the  theory. 

The  direction  of  the  wind  seems  to  have  little  effect  on  the 
formation  of  cusps.  They  have  been  observed  in  process  of 
formation  during  onshore,  offshore,  and  longshore  winds,  both 
gentle  and  fairly  strong.  Under  ordinary  conditions  the  only 
result  noticed  was  a  more  or  less  marked  cliffing  on  one  side  of 
the  cusps  when  the  wind  produced  small  waves  at  an  angle 
oblique  to  the  beach.  The  cusps  thus  cliff ed  may  have  been 
partially  developed  before  the  oblique  waves  began  their  work. 
If  the  wind  is  strong  enough  and  from  such  a  direction  as  to 
combine  with  the  breakers  in  producing  a  very  irregular  wave 
attack,  the  formation  of  cusps  is  probably  interferred  with,  since 
numerous  observations  tend  to  show  that  a  fairly  regular  ad- 
vance and  retreat  of  the  water  is  essential  to  their  development. 

Beach  cusps  are  formed  at  all  stages  of  the  tide.  It  is  prob- 
able that  the  greatly  elongated  type  is  produced  when  the  waves 
remain  of  approximately  the  same  size  during  a  falling  tide,  but 


BEACH  CUSPS  475 

the  development  of  this  type  has  not  been  observed  throughout 
the  entire  process. 

The  direction  of  wave  advance  has  been  carefully  noted  where- 
ever  cusps  were  being  formed.  On  the  basis  of  numerous  ob- 
servations on  all  kinds  of  beaches  and  of  extended  experimenta- 
tion, it  may  be  confidently  stated  that  the  best  conditions  for 
cusp  formation  exist  when  a  single  series  of  waves  advances 
parallel  with  the  beach.  It  is  possible  that  cusps  may  be  pro- 
duced by  waves  striking  the  shore  at  a  markedly  oblique  angle, 
but  no  satisfactory  evidence  that  such  is  the  case  has  been 
secured.  On  the  other  hand,  the  progressive  destruction  of 
cusps  by  oblique  waves  has  been  repeatedly  observed.  Such 
partially  destroyed  forms  are  shown  in  the  lower  left-hand  corner 
of  Figure  141.  I  am  inclined  to  think  that  the  asymmetrical 
"  cusplets "  reported  by  Wilson'^  were  formerly  symmetrical 
beach  cusps  of  the  ordinary  type,  which  were  later  cliffed  by  the 
oblique  waves  shown  in  a  photograph  reproduced  in  his  paper. 
Intersecting  waves  of  the  type  appealed  to  by  Branner  have 
been  seen  in  a  number  of  cases,  but  no  cusps  have  been  observed 
to  develop  under  the  action  of  such  waves. 

The  periodicity  of  the  waves  does  not  appear  to  be  a  signifi- 
cant factor  in  beach  cusp  formation.  Varying  the  period  with 
artificial  waves  produces  no  apparent  effect  on  the  cusps. 

Jefferson^-  says:  "  The  cusps  seem  related  to  a  longshore 
current,  their  precise  cause  not  being  evident";  but  he  does 
not  indicate  in  what  manner  the  cusps  seemed  related  to  the 
current.  In  most  of  my  observations  no  evidence  of  a  long- 
shore movement  of  the  water  was  found.  In  the  few  cases 
where  a  distinct  drift  or  current  in  one  direction  was  apparent 
there  seemed  to  be  no  relation  between  the  current  and  the 
cusps.  Beach  cusps  seem  clearly  to  be  the  product  of  on-  and 
offshore  movements  of  the  water. 

Artificial  Beach  Cusps.  —  From  the  observation  of  natural 
beach  cusps  in  process  of  formation  the  conclusion  was  reached 
that  cusps  could  be  formed  by  a  single  series  of  waves  advanc- 
ing parallel  with  the  shore.  In  order  to  test  the  validity  of  this 
conclusion  the  artificial  production  of  cusps  was  attempted.  A 
sand  beach  was  constructed  along  one  side  of  a  tank  5  feet  square 
and  the  water  in  the  tank  raised  until  it  rested  against  the 
beach  slope.     To  make  that  slope  as  smooth   and  gentle  as 


476 


MINOR  SHORE  FORMS 


possible,  large  waves  were  washed  over  the  beach  until  it  ap- 
peared to  the  eye  as  a  perfectly  uniform,  gentle  slope  of  sand. 
On  the  opposite  side  of  the  tank  from  the  beach  was  arranged 
the  wave-producing  apparatus.  This  consisted  at  fu-st  of  a 
board  which  was  tipped  up  and  down  by  hand;  later  of  two 
boards  hinged  together,  one  of  which  was  made  stationary  on 
the  floor  of  the  tank,  while  the  other  could  be  raised  and  lowered 
by  a  long  handle  connecting  with  its  free  edge.  With  this 
simple  apparatus  it  was  possible  to  propel  on  the  beach  a  series 
of  parallel,  straight  waves,  varying  in  size  and  periodicity  as 
the  experimenter  desired.  It  was  found  that  beach  cusps  re- 
sembling closely  those  in 
nature  could  be  artificially 
produced  (Fig.  144).  The 
characteristic  features  of 
these  artificial  cusps  have 
lieen  discussed  above. 

Theories  of  Origin.  — 
With  the  characteristics 
of  beach  cusps  in  mmd, 
we  may  critically  examine 
the  theories  which  have 
been  proposed  to  account 
for  their  origin. 

The  unpublished  manu- 
script of  1887,  in  which 
Lane  discusses  the  characteristics  of  beach  cusps,  does  not  set  forth 
a  complete  theory  of  their  origin,  but  does  contain  exceptionally 
good  observations  on  the  more  significant  features  of  their 
occurrence.  It  will  presently  appear  that  some  of  the  signifi- 
cant relationships  noted  by  Lane,  and  quoted  on  an  earlier  page, 
are  necessarily  involved  in  the  theory  of  origin  advanced  by  the 
present  writer. 

According  to  Shaler,  "  the  inner  edge  of  the  swash  line  .  .  . 
has  a  very  indented  front,  due  to  the  fact  that  it  is  shaped  by 
a  criss-cross  action  of  many  different  waves^^."  The  projecting 
tongues  of  water  push  back  the  pebbles,  leaving  indentations 
or  bays,  which  are  then  enlarged  under  the  continued  wave 
attack  during  the  rising  tide.  It  should  be  noted,  however, 
that  the  indentations  of  the  inner  edge  of  the  swash  line  on  a 


Fig.  144.  —  Artiticiai  beach  cusps. 


BEACH  CUSPS  477 

smooth  beach  are  extremely  irregular,  and  vary  in  position  with 
every  wave  advance  until  the  development  of  cusps  and  inter- 
cusp  depressions  affords  more  definite  guidance.  That  a  single 
advance  of  the  irregular  inner  edge  of  the  swash  could  develop 
bays  which  would  thereafter  control  the  action  of  the  waves 
seems  doubtful.  The  inner  edge  of  the  swash  is  thin  as  well  as 
irregular  and  variable,  and  under  these  conditions  must  be  very 
ineffective  in  developing  intercusp  spaces  or  "  bays."  'Nor  does 
the  theory  as  stated  by  its  author  explain  the  regularity  in  spac- 
ing of  the  cusps  nor  their  respacing  consequent  upon  a  change 
in  size  of  waves.  It  would  seem  that  Shaler's  theory  does  not 
go  far  enough  adequately  to  explain  the  observed  phenomena. 

In  the  account  of  "  ridges  and  furrows  "  (cusps  and  intercusp 
spaces)  given  by  Cornish^^  it  is  stated  that  the  water  washes 
depressions  at  selected  places  because  neither  the  force  of  the 
water  nor  the  resistance  of  the  beach  material  to  erosion  is 
absolutely  uniform.  The  regular  spacing  of  the  cusps  is  not 
explained,  nor  does  the  author  appear  to  have  recognized  this 
character  of  their  distribution.  Neither  does  he  recognize  the 
fact  that  gentle  waves  build  cusps.  The  erosion  which  pro- 
duces the  "  furrowing  "  is  related  by  him  to  a  change  from 
small  to  large  waves  only.  But  we  have  seen  that  cusps  form 
under  reverse  conditions  as  well.  It  thus  appears  that  Cornish 
points  out  certain  causes  of  the  unequal  erosion  of  beaches,  but 
does  not  throw  much  light  upon  the  origin  of  the  cusps. 

The  seaweed  barrier  theory  of  Jefferson-^  advanced  to  account 
for  the  occurrence  of  cusps  on  a  beach  where  there  happened 
to  be  considerable  accumulations  of  seaweed  at  the  time,  breaks 
down  under  the  test  of  a  broader  application.  There  are  also 
serious  objections  to  the  theory  aside  from  the  fact  that  cusps 
are  abundantly  developed  on  beaches  free  from  seaweed  and 
other  similar  material.  Even  if  we  admit  that  a  strip  of  sea- 
weed might  form  an  effective  dam  behind  which  considerable 
masses  of  water  would  be  imprisoned,  we  must  regard  it  as  in 
the  highest  degree  improbable  that  this  water  would  break 
through  the  seaweed  barrier  at  a  large  number  of  rather  evenly 
and  often  closely  spaced  intervals.  The  degree  of  regularity 
in  beach  cusp  spacing  is  wholly  incompatible  with  the  seaweed 
barrier  theory. 

On  the  other  hand  it  should  be  remembered  that  after  the 


478  MINOR  SHORE   FORMS 

cusps  have  once  formed,  a  seaweed  barrier,  as  well  as  a  barrier 
of  sand  or  gravel,  may  be  breached  by  the  waves  where  their 
water  is  concentrated  for  the  attack  in  the  intercusp  spaces. 
Thus  an  observer  might  find  breaches  in  the  barrier  correspond- 
ing with  the  intercusp  spaces.  As  shown  more  fully  on  a  pre- 
ceding page,  both  theoretical  considerations  and  the  field  evi- 
dence support  the  view  that  the  breaching  is  effected  by  direct 
wave  attack,  and  not  by  the  escape  of  water  imprisoned  behind 
the  barrier.  There  is  good  ground  for  the  belief  that  the  breach- 
ing of  the  seaweed  barrier  on  Lynn  Beach  was  the  effect  instead 
of  the  cause  of  cusp  formation. 

In  Jefferson's  more  recent  accounts"^  the  question  of  origin 
is  very  briefly  referred  to;  but  from  such  reference  it  appears 
that  the  author  later  considered  a  barrier  of  sand  or  gravel 
capable  of  playing  the  same  role  in  cusp  formation  as  a  seaweed 
barrier.  It  is  further  implied  that  other  cusps  must  have  had 
a  different  but  unknown  origin.  The  objections  urged  against 
the  seaweed  barrier  theory  apply,  in  the  main,  with  equal  force 
against  the  sand  or  gravel  Ijarrier  theory.  It  is  true  that  ridges 
of  sand  and  gravel  are  more  frequent  on  beaches  than  barriers 
of  seaweed;  but  the  evidence  is  conclusive  that  cusps  are  formed 
when  such  ridges  are  absent,  and  that  even  when  present  such 
ridges  are  breached  from  the  seaward  side  by  direct  wave  attack, 
and  not  from  the  landward  side  by  impounded  waters. 

On  both  natural  and  artificial  beaches  more  or  less  distinct 
ridges  are  sometimes  broken  through  before  any  distinct  cusps 
have  been  formed.  This  led  me  to  entertain  the  hypothesis 
that  direct  wave  attack  on  a  fairly  uniform  ridge  would  develop 
breaches  in  the  ridge  at  intervals  proportional  to  the  size  of  the 
waves.  It  seems  probable,  however,  that  faint  undulations  in 
the  beach,  on  the  seaward  side  of  the  ridge,  may  help  to  deter- 
mine the  points  of  breaking  just  as  the  more  evident  cusps  and 
intercusp  spaces  do  in  other  cases,  and  that  the  breached  ridges 
are  therefore  but  one  phase,  and  not  an  essential  one,  of  the 
process  of  cusp  formation,  as  explained  on  a  later  page. 

Branner's  theory",  while  very  suggestive,  seems  to  present 
insuperable  obstacles,  as  will  be  apparent  on  the  inspection  of 
his  diagrams  (Figs.  139  and  140).  The  hypothetical  wave  lines 
are  evenly  spaced,  and  the  wave  length  in  both  sets  is  the 
same.     This   is   a   condition   which  probably  never  obtains  in 


BEACH  CUSPS  4:79 

nature,  and  yet  such  an  improbable  condition  is  an  essential 
element  of  the  theory.  If  the  two  sets  of  waves  are  given 
different  wave  lengths,  or  if  one  set  of  waves  has  a  velocity 
differing  from  that  of  the  other,  or  if  either  set  of  waves  is  irregu- 
larly spaced,  then  the  points  of  wave  interference  will  not  reach 
the  beach  at  the  same  place  twice  in  succession.  If  we  endeavor 
to  approximate  natural  conditions  by  introducing  any  one  of 
the  three  types  of  irregularities  mentioned  (and  probably  all 
three  exist  in  every  case  of  intersecting  waves),  we  must  correct 
the  diagrams  by  making  the  dotted  lines  meet  the  shoreline  at 
every  conceivable  point.  This  done,  the  supposed  reason  for 
cusp  formation  disappears. 

It  has  been  shown  on  preceding  pages  that  the  physical  con- 
ditions necessary  for  cusp  formation  exist  in  parallel  waves. 
One  might  accordingly  surmise  that  in  intersecting  waves  the 
necessary  equilibrium  would  be  destroyed  and  the  formation  of 
cusps  rendered  more  difficult,  or  even  impossible.  I  believe 
this  to  be  the  case.  In  1907,  while  camping  near  Huletts 
Landing,  opportunity  was  afforded  to  make  numerous  obser- 
vations during  a  period  of  six  weeks,  on  a  portion  of  the  lake 
shore  where  intersecting  waves  were  usually  developed  by  a 
sand  and  gravel  bar  offshore.  At  no  time  were  cusps  observed 
on  the  portion  of  the  beach  where  intersecting  waves  arrived, 
although  they  were  frequently  found  on  adjacent  portions. 
These  observations  led  to  the  belief  that  intersecting  waves 
tend  to  prevent  rather  than  to  cause  the  formation  of  beach 
cusps; 

Inasmuch  as  the  "  cusplets  "  described  by  Wilson^^  appear 
to  be  true  beach  cusps  of  somewhat  unusual  form,  it  is  proper 
to  consider  the  hypothesis  offered  to  account  for  their  origin. 
According  to  this  author,  evenly  spaced  waves  striking  a  straight 
shoreline  at  an  oblique  angle  will  give  evenly  spaced  points  of 
wave-breaking  at  which  cusps  will  develop.  Because  at  any 
given  instant  a  series  of  oblique  waves  will  be  breaking  at  a 
number  of  different  points  along  a  beach,  the  author  assumes 
that  the  points  of  simultaneous  wave-breaking  will  be  nodal 
points  where  material  will  tend  to  accumulate.  It  would 
appear  that  no  account  is  taken  of  the  fact  that  every  oblique 
wave  of  the  series  breaks  not  only  at  the  point  observed  during 
a  given  instant,  but  also  at  all  the  other  points  up  and  down  the 


480  MINOR  SHORE  ^ORMS 

beach,  so  long  as  the  wave  exists.  The  point  of  breaking  of  an 
obUque  wave  sweeps  along  the  shore  until  the  end  of  the  wave 
itself  is  reached.  In  a  series  of  waves  parallel  to  each  other, 
but  oblique  to  the  shoreline,  each  wave  in  turn  breaks  continu- 
ously from  one  end  of  the  beach  to  the  other.  Under  these 
conditions  no  nodal  points  can  develop,  and  the  fact  that  the 
waves  are  a  given  distance  apart,  and  that  at  any  given  instant 
their  points  of  contact  with  the  shore  are  evenly  spaced,  is 
immaterial  so  far  as  the  distribution  of  force  of  wave  attack  is 
concerned. 

In  addition  to  the  theoretical  objections  to  Wilson's  theory 
must  be  added  the  observed  fact  that  oblique  waves  appear  to 
be  much  less  favorable  to  cusp  formation  than  are  waves  parallel 
to  the  shoreline.  Oblique  waves  have  been  observed  in  the 
process  of  cliffing  the  sides  of  cusps  exposed  to  their  attack, 
and  the  remains  of  the  cusps  then  have  the  asymmetrical  form 
decribed  by  this  author. 

In  attempting  to  explain  the  formation  of  beach  cusps  I  have 
tested  and  rejected  several  working  hypotheses  in  addition  to 
those  mentioned  above.  For  example,  there  was  considered  the 
possibility  that  the  waves  breaking  parallel  with  the  shore  had 
superposed  obliquely  upon  them  smaller  waves,  and  that  the 
portions  of  the  main  waves  thus  increased  in  height  excavated 
the  intercusp  spaces.  One  bit  of  evidence  which  appeared  to 
harmonize  with  this  theory  was  personally  reported  to  me  by 
Mr.  T.  I.  Read,  who  noted  that  on  Virginia  Beach  the  incoming 
waves  showed  the  first  tendency  to  break  at  regularly  spaced 
intervals  which  corresponded  with  the  intervals  between  cusps. 
The  hypothesis  was  rejected  because  the  cause  was  irregular, 
while  the  effect  was  regular;  because  of  an  almost  complete 
lack  of  direct  evidence  pointing  to  a  relation  between  superposed 
waves  and  cusps;  and  because  experiments  seemed  to  point 
conclusively  to  some  other  origin. 

Another  hypothesis  was  based  on  the  assumptjon  that  an 
extended  sheet  of  water  descending  an  inclined  plane  may  not 
move  with  the  same  velocity  throughout,  but  may  tend  to  de- 
velop lines  of  swifter  flow,  or  currents,  at  certain  intervals.  I 
was  tempted  to  make  this  assumption  because  of  the  fact  that 
water  descending  a  flat-bottomed  inclined  trough,  or  conduit, 
does  not  flow  uniformly,  but  is  successively  retarded  in  such  a 


BEACH   CUSPS  481 

manner  as  to  produce  a  succession  of  waves.  Admirable  illus- 
trations of  this  phenomenon  have  been  published  by  Cornish^^ 
in  a  paper  on  ''  Progressive  Waves  in  Rivers."  It  occurred  to 
me  that  if  a  l^roader  sheet  of  fluid  were  retarded  by  friction 
while  descending  an  inclined  plane,  the  resistance  might  be 
overcome  fu'st,  or  more  rapidly,  at  certain  points,  and  that  the 
slightly  increased  rate  of  advance  at  these  points  would  disturb 
the  equilibrium  in  such  manner  as  to  create  zones  or  currents 
of  accelerated  flow  wherever  these  slight  initial  advantages  had 
been  gained.  If  the  sheet  of  water  were  shallow,  there  would 
be  a  tendency  for  the  currents  to  be  smaller  and  more  closely 
spaced  than  if  the  sheet  of  water  were  of  greater  depth.  This 
hypothesis  was  especially  tempting,  inasmuch  as  granting  the 
basal  assumption  all  the  phenomena  of  beach  cusps  find  a 
ready  explanation.  Small  waves  advancing  and  retreating  on 
the  beach  would  give  small  currents  closel}^  spaced,  which  would 
in  turn  scour  small  intercusp  spaces  leaving  closely  spaced  cusps. 
Any  change  in  the  size  of  waves  resulting  in  a  change  in  the  size 
and  spacing  of  the  currents  would  necessitate  a  respacing  of 
the  cusps.  The  hypothesis  does  not  lack  support  so  far  as  the 
phenomena  of  beach  cusps  are  concerned,  but  it  is  based  on  an 
assumption  which  does  lack  support.  I  have  questioned  a  num- 
ber of  engineers  and  physicists  in  regard  to  the  matter,  but 
could  learn  nothing  favorable  to  the  assumption. 

The  hypothesis  which  best  accords  with  all  of  the  available 
evidence  may  now  be  set  forth.  Concisely  stated,  it  is  that 
selective  erosion  by  the  swash  develops  from  initial  irregular 
depressions  in  the  beach  shallow  troughs  of  approximately  uni- 
form breadth,  whose  ultimate  size  is  proportional  to  the  size 
of  the  waves,  and  determines  the  relatively  uniform  spacing 
of  the  cusps  which  develop  on  the  inter-trough  elevations.  This 
theory  differs  essentially  from  those  proposed  by  Branner  and 
Wilson  in  that  neither  intersecting  nor  oblique  waves  are  ap- 
pealed to  and  the  spacing  of  the  waves  is  disregarded;  from 
those  proposed  by  Jefferson  and  Cornish  in  that  the  cusps  are 
not  regarded  as  mere  erosion  remnants  of  a  once  continuous 
ridge,  while  uniformity  of  spacing  depending  on  wave  size  is 
considered  of  vital  importance;  from  the  theory  proposed  by 
Shaler  in  that  no  importance  is  attached  to  the  irregular  front 
of  the  swash,  the  ability  of  the  thin  edge  of  the  swash  to  develop 


482  MINOR  SHORE  FORMS 

the  intercusp  bays  is  not  admitted,  while  the  size  of  the  wave 
is  correlated  with  the  width  of  intercusp  spaces.  Other  points 
of  difference  will  appear  in  the  explanation  which  follows. 

Every  beach  contains  numerous  inequalities  which  tend  to 
prevent  a  uniform  flow  of  water  up  and  down  the  beach  during 
wave  action.  These  inequalities  have  a  variety  of  causes.  Sur- 
face run-off  after  rains  may  develop  channels  on  the  beach;  the 
water  draining  out  of  the  sand  at  the  upper  part  of  the  beach 
after  high  tides  or  after  high  waves  may  produce  the  same 
result.  Pebbles  lying  on  a  sandy  beach  interfere  with  the 
swash  of  water  up  and  down  the  beach,  and  cause  some  channel- 
ing. The  waves  are  never  even-crested,  and  may  be  very  irreg- 
ular if  oblique  waves  are  superposed  on  them;  the  irregularity 
of  the  swash  line,  mentioned  by  Shaler,  may  initiate  irregu- 
larities on  the  beach.  Remnants  of  old  beach  cusps,  not  wholly 
obliterated,  form  another  source  of  irregularity;  and  still  other 
sources  might  be  mentioned. 

The  continual  swashing  of  the  water  up  and  down  the  beach 
tends  to  enlarge  the  irregular  depressions  over  which  the  water 
passes.  Larger  channels  are  better  adapted  to  the  movements 
of  the  large  volumes  of  wave-supplied  water.  It  is  inevitable 
that  in  the  enlarging  of  some  depressions  others  will  be  obliter- 
ated, just  as  in  the  case  of  growing  drainage  basins  many  small 
basins  disappear  as  independent  features,  while  the  few  increase 
in  size.  Those  depressions  on  the  beach  which  develop  to  larger 
proportions  will  be  the  ones  which  have  some  initial  accidental 
advantage,  and  which  increase  that  advantage  as  they  grow; 
just  as  the  accidentally  favored  drainage  basins  increase  in 
size  and  advantage  at  the  expense  of  those  which  began  the 
contest  with  but  a  slightly  less  favorable  chance.  The  tendency 
of  wave  action  will  be  to  develop  from  initial  irregularities  a 
smaller  number  of  broad  and  shallow  depressions  on  that  por- 
tion of  the  beach  traversed  by  the  swash.  The  depressions  will 
be  broad,  because  they  are  thus  better  adapted  to  the  move- 
ments of  large  volumes  of  water;  and  shallow,  because  the 
elevations  between  the  depressions  are  also  buried  under  the 
advancing  and  retreating  waters  and  are  kept  worn  down  to  a 
moderate  height.  Only  near  the  upper  zone  of  wave  action, 
where  the  water  invades  the  depressions  but  does  not  rise  high 
enough  to  override  the  intervening  elevations,  are  the  depres- 


BEACH   CUSPS  483 

sions  continually  scoured  deeper  and  the  unworn  elevations  left 
as  pronounced  ridges.  Out  toward  the  seaward  margin  of  the 
submarine  terrace,  deposition  rather  than  erosion  prevails,  and 
the  delta  scallops  may  rise  higher  than  the  seaward  extension 
of  the  elevations  which  exist  farther  up  the  beach. 

There  is  a  limit  to  the  width  to  which  the  depressions,  or 
shallow  "  channels,"  if  we  may  so  call  them,  can  develop.  In- 
asmuch as  the  enlargement  of  some  necessitates  the  obliteration 
of  others,  enlargement  will  continue  only  so  long  as  the  impulse 
toward  growth  imposed  on  the  more  favored  channels  is  suffi- 
ciently great  to  overcome  the  tendency  of  their  neighbors  to 
enlarge.  Equilibrium  will  be  established  when  adjacent  chan- 
nels are  of  approximately  the  same  size,  and  at  the  same  time 
of  a  size  appropriate  to  the  volumes  of  water  traversing  them. 
If  the  waves  are  low  and  the  volumes  of  water  consequently 
inconsiderable,  equilibrium  will  be  reached  while  the  channels  are 
yet  small.  But  if  the  waves  are  high  and  the  volumes  of  water 
large,  a  perfect  adjustment  will  not  be  reached  until  the  chan- 
nels have  attained  great  size. 

The  remainder  of  the  process  is  easily  understood.  With 
th(;  water  advancing  repeatedly  up  a  beach  which  is  faintly  but 
systematically  channeled,  as  above  indicated,  there  will  be  a 
constant  tendency  to  push  gravel  and  other  debris  farther  up 
the  slope  in  the  depressed  areas  than  in  the  intervening  areas. 
Near  the  upper  limit  of  wave  action  the  depressed  areas  alone 
are  invaded  by  water  and  are  scoured  deeper  as  the  gravels  are 
pushed  back  and  the  finer  material  dragged  down  to  form  the 
delta  scallops.  The  intervening  areas  are  fashioned  into  beach 
cusps,  whose  sharpened  points  divide  the  waters  of  the  advanc- 
ing waves  and  concentrate  the  attack  toward  the  heads  of  the 
depressions.  The  coarse  material  is  constantly  pushed  into  the 
cusp  areas,  the  channels  swept  relatively  clean.  With  a  rising 
tide  both  channels  and  cusps  are  pushed  progressively  up  the 
beach;  with  a  falling  tide  some  of  the  gravels  may  be  dragged 
downward  to  give  much  elongated  cusps. 

There  ar?  a  number  of  considerations  which  appear  to  sup- 
port the  foregoing  theory  of  beach  cusp  formation.  The  theory 
accounts  for  the  degree  of  regularity  observed  in  the  spacing  of 
beach  cusps,  since  the  spacing  is  dependent  on  the  development 
c:  channels  which  do  not  reach  equilibrium  until  of  approxi- 


484 


MINOR  SHORE   FORMS 


mately  uniform  size.  At  the  same  time  the  considerable  degree 
of  irregularity  in  spacing  occasionally  observed  is  not  incom- 
patible with  the  theory,  since  the  degree  of  regularity  in  spacing 
depends  on  the  progress  which  has  been  made  toward  the  estab- 
lishment of  perfect  equilibrium.  The  occurrence  of  imperfect 
and  compound  cusps  is  readily  explained  as  the  product  of  wave 
action  in  channels  not  yet  eroded  to  the  standard  size,  as  when 
two  unusually  small  channels  have  not  yet  been  fashioned  into 
a  single  large  one,  and  consequently  give  a  compound  cusp  (Fig. 
145)  near  their  upper  limits.  We  should  expect,  on  the  basis  of 
this  interpretation,  that  irregular  and  compound  cusps  should  be 
most  characteristic  of  the  early  stages  of  development,  and  the 


VoWV 


J.?..- 


mM0ln\\ 


,0    a  •,•••<) 


Fig.  145.  —  Beach  cusps  (after  Jefferson)  showing  compound  cusps  at  right. 


experiments  with  artificial  cusps  prove  most^  conclusively  that 
this  is  the  case.  One  of  the  commonest  occurrences  in  the 
experiments  is  the  gradual  moulding  of  irregular  and  compound 
cusps  into  simple  cusps  regularly  spaced. 

The  respacing  of  cusps  with  a  change  in  size  of  waves  may  be 
thus  explained:  A  given  set  is  formed  and  driven  up  the  beach, 
and  then  left  by  the  falling  tide.  The  size  of  waves  changes, 
and  new  channels  appropriate  to  them  are  formed.  New  cusps 
result,  and  as  the  tide  rises  these  are  in  turn  pushed  up  the  beach. 
If  the  new  cusps  do  not  coincide  in  position  with  the  older  ones, 
when  the  latter  are  reached  their  ends  will  be  eroded  by  the 
waters  converging  on  them  from  between  the  new  ones.  Repe- 
titions of  this  process,  with  waves  of  decreasing  size,  will  give 
several  sets  of  partially  preserved  cusps,  each  set  more  closely 
spaced  than  the  set  above  it.  On  the  other  hand,  if  a  big 
storm  drives  in  unusually  high  waves,  big  channels  will  be  formed, 


BEACH  CUSPS  485 

older  sets  of  cusps  will  be  quickly  swept  out  of  existence,  and 
a  single  set  of  large,  widely  spaced  cusps  will  be  de- 
veloped. 

In  the  laboratory  experiments  difficulty  was  often  experienced 
in  getting  the  cusps  started.  The  artificial  beach  was  very 
smooth,  of  fairly  uniform  sand  grains.  It  appeared  that  the 
difficulty  was  due  to  the  regularity  of  the  beach,  on  account  of 
which  the  initiation  of  channels  was  delayed.  In  order  to 
facilitate  the  process  a  series  of  closely  spaced  creases  down 
the  beach  was  made,  after  which  the  cusps  began  to  form  more 
rapidly.  As  already  shown,  the  artificial  creases  did  not  control 
the  number  or  position  of  the  cusps  and  their  intervening  spaces, 
but  the  importance  of  initial  depressions  in  the  cusp-making 
process  seemed  clearly  indicated. 

On  Westquage  Beach,  Rhode  Island,  the  writer  has  watched 
a  series  of  parallel  "  creases,"  or  rill  lines,  without  any  associ- 
ated cusps,  develop  into  channels  or  intercusp  spaces  with 
fairly  good  associated  sand  cusps.  Such  observations  are  rela- 
tively rare,  however,  probably  because  the  initial  irregularities 
are  often  indistinct  undulations  in  the  beach  surface  or  are 
soon  transformed  into  such  undulations;  and  because  the  succes- 
sive changes  in  the  form  of  broad,  shallow  channels  on  a  gravel 
or  sand  and  gravel  beach  are  difficult  to  trace.  The  "  ribbed  " 
structure  occasionally  reported  by  observers  looking  for  cusps 
probably  represents  an  early  stage  of  cusp  formation. 

The  tendency  of  intersecting  or  criss-cross  waves  would  be 
continually  to  shift  the  sands  first  in  one  direction  and  then  in 
another  obliquely  over  the  beach,  and  thus  to  prevent  the  forma- 
tion of  systematic  channels.  This  would  account  for  the  ob- 
served failure  of  such  waves  to  form  beach  cusps,  although  they 
might  attack  cusps  previously  formed,  or  leave  a  beach  with 
irregularities  which  might  affect  the  formation  of  later  cusps. 

In  a  similar  way,  to  a  less  extent,  a  single  series  of  oblique 
waves  would  not  seem  favorable  to  cusp  formation,  because  of 
the  lateral  element  in  the  movement  of  the  water,  which  would 
continually  tend  to  wash  the  interchannel  elevations  into  the 
channels,  and  so  to  fill  them  up. 

It  is  not  necessary  to  review  all  the  details  of  beach  cusp  char- 
acteristics in  connection  with  the  theory  set  forth  above.  It  is 
sufficient   to  state   that   the   author   has  found   no  feature   of 


486  MINOR  SHORE  FORMS 

beach  cusps  which  is  incompatible  with  the  theory,  while  the 
assumed  conditions  of  wave  action  appear  to  rest  on  a  reason- 
able basis. 

LOW   AND   BALL 

The  shoreface  zone,  or  possibly  the  inner  margin  of  the  offshore 
zone,  is  frequently  characterized  by  submarine  bars  or  ridges, 
separated  by  distinct  longitudinal  depressions  and  lying  par- 
allel to  the  shoreline.  English  writers  apply  the  name  hall  to 
the  ridges  and  low  to  the  depressions.  The  continuity  of  the 
ball  is  sometimes  truly  remarkable,  RusselP**  describing  exam- 
ples on  the  shores  of  Lake  Michigan  which  "  can  be  traced 
continuously  for  hundreds  of  miles."  In  this  case  "  there  are 
usually  two,  but  occasionally  three,  distinct  sand  ridges;  the 
first  being  about  200  feet  from  the  land,  the  second  75  or  100 
feet  beyond  the  first,  and  the  third,  when  present,  about  as  far 
from  the  second  as  the  second  is  from  the  first.  Soundings  on 
these  ridges  show  that  the  first  has  about  8  feet  of  water  over  it, 
and  the  second  usually  about  12;  between,  the  depth  is  from 
10  to  14  feet  ....  They  follow  all  the  main  curves  of  the 
shore,  without  changing  their  character  or  having  their  con- 
tinuity broken."  Russell  suggests  that  these  balls  may  repre- 
sent accumulations  of  shore  debris  along  the  lines  where  the 
undertow  loses  its  force  during  storms  of  varying  degrees  of 
intensity;  but  qualifies  the  suggestion  with  the  statement  that 
"  the  complete  history  of  these  structures  has  not  been  deter- 
mined." 

The  balls  of  Lake  Michigan  were  earlier  described  by  Desor^i, 
who  in  1851  attributed  them  to  transportation  and  deposition 
by  ''  currents,"  and  stated  his  belief  that  the  elevated  beaches 
about  the  Great  Lakes  were  really  submarine  bars  of  the  same 
type  which  had  been  exposed  to  view  by  a  rising  of  the  land. 
Whittlesey^-  treats  them  briefly  as  a  product  of  "  lateral  cur- 
rents." In  1870  Andrews^^  called  attention  to  the  "  subaqueous 
ridge  or  bar "  which  is  "  uniform  in  all  the  sand  shores "  at 
the  head  of  Lake  Michigan.  Gilbert  at  first^^  considered  the 
balls  of  the  Great  Lakes  region  as  barrier  beaches  or  spits 
built  at  the  lake  surface  and  later  submerged  by  a  rise  of  the 
waters;  but  later^^  decided  that  they  were  originally  formed  as 
subaqueous  bars.     Concerning  the  method  of  their  formation 


LOW  AND   BALL  487 

he  writes:  "  Under  conditions  not  yet  apparent,  and  in  a  man- 
ner equally  obscure,  there  is  a  rhythmic  action  along  a  certain 
zone  of  the  bottom.  That  zone  lies  lower  than  the  trough 
between  the  greatest  storm  waves,  but  the  water  upon  it  is 
violently  oscillated  by  the  passing  waves.  The  same  water  is 
translated  lakeward  by  the  undertow,  and  the  surface  water 
above  it  is  translated  landward  by  the  wind,  while  both  move 
with  the  shore  current  parallel  to  the  beach.  The  rhythm  may 
be  assumed  to  arise  from  the  interaction  of  the  oscillation,  the 
landward  current,  and  the  undertow." 

The  earliest  description  of  low  and  ball  of  which  I  find  reccrd 
is  given  by  Hagen  in  his  "  Handbuch  der  Wasserbaukunst^^." 
Hagen  considers  the  phenomenon  a  normal  characteristic  of  a 
gently  sloping  sea-bottom,  and  refers  to  a  popular  belief  that 
three  parallel  balls  {"  Riffe  ")  are  always  found  in  association. 
He  shows,  however,  that  the  number  is  not  constant,  as  many 
as  five  sometimes  being  revealed  by  careful  soundings.  The 
ridge  nearest  the  shore  is  highest,  those  farther  out  progres- 
sively decreasing  in  altitude  until  the  outermost  may  rise  an 
almost  imperceptible  distance  above  the  sea  floor.  In  Hagen's 
opinion  the  ridges  form  where  on-coming  waves  meet  the  under- 
tow, especially  where  the  undertow  is  reinforced  by  backward 
moving  water  of  normal  oscillatory  waves. 

A  brief  account  of  the  form  of  parallel  balls  is  given  in  Braun's 
"  Entwickelungsgeschichtliche  Studien  an  europaischen  Flach- 
landskiisten  und  ihren  Diinen,"  under  the  caption  "Das  San- 
driff^^."  He  follows  Lehmann'^  in  considering  the  ball  as  a 
forerunner  of  the  offshore  bar  or  beach  ridge,  the  ball  being 
driven  landward  and  ultimately  raised  above  sealevel  by  the 
action  of  the  waves.  Observations  of  European  examples  lead 
to  the  conclusion  that  normally  the  landward  side  of  the  ball  is 
steeper  than  the  seaward  slope.  Otto,  on  the  contrary,  in  a 
full  description  of  these  submerged  ridges  published  in  his  work 
on  "  Der  Darss  und  Zingst^^ "  finds  them  more  variable  in  form 
and  in  behavior.  They  are  sometimes  evenly,  sometimes  irregu- 
larly spaced,  and  often  migrate  seaward  as  well  as  landward. 
Sudden  and  marked  changes  in  the  ridges  occur  only  with 
great  storms.  A  comparison  of  wave  lengths  and  the  distances 
between  ridges  shows  that  no  correspondence  exists  between 
the  two  measurements.     Both   Braun  and  Otto  give  a  short 


488  MINOR  SHORE  FORMS 

bibliography  of  the  subject,  which  should  be  consulted  by  those 
desirous  of  securing  further  data  regarding  the  lows  and  balls  of 
the  Baltic  shores  and  other  coasts  of  continental  Europe. 

Under  the  title  "  Low  and  Ball  of  a  Sand}-  Shore^","  Cornish 
states  that  the  building  up  of  a  "  full  "  of  sand  in  front  of  the 
breaker  is  accompanied  by  the  excavation  of  a  trough,  at  the 
back  of  the  breaker.  Beyond  the  trough  there  rises  a  sandbank 
which  is  called  the  ball,  while  the  trough  itself  is  the  low.  Ebb 
tide  may  reveal  the  surface  of  the  ball,  under  which  condition  a 
lagoon  occupies  the  low  between  the  ball  and  the  beach. 
Wheeler^^  also  speaks  of  the  low  as  a  gully  running  parallel  to 
the  coast  cut  by  the  action  of  breakers,  and  is  of  the  opinion 
that  the  ball  may  rise  permanently  above  the  water  surface, 
causing  a  permanent  lagoon  or  shallow  creek  in  the  adjacent  low. 

Kemp*2  has  recorded  some  valuable  observations  regarding 
the  lows  and  balls  of  the  Florida  east  coast.  At  Melbourne 
Beach,  and  for  an  indefinite  distance  north  and  south,  the  shore 
is  normally  bordered  by  a  distinct  channel  varying  in  breadth 
from  15  to  60  yards  and  usually  not  so  deep  but  that  bathers 
could  walk  across  it  to  the  bar  beyond  at  low  tide.  The  crest  of 
the  bar  rose  within  a  few  inches  of  the  water  surface,  but  was 
never  seen  exposed.  Those  engaged  in  surf-fishing  for  "channel 
bass"  become  familiar  with  all  changes  in  the  low,  for  this  is  the 
channel  in  which  the  bass  run.  After  maintaining  a  fairly  con- 
stant position  for  three  months  in  the  winter  of  1915-16,  the  bar 
migrated  shoreward  under  the  influence  of  heavy  surf  from  a 
strong  easterly  gale.  After  the  storm  died  down  the  bar  con- 
tinued its  shoreward  progress  until  the  low  was  reduced  to  a 
breadth  of  5  yards,  then  2  yards,  and  finally  was  extinguished. 
The  next  fall  the  fishermen  found  a  new  bar  with  broad  channel 
intervening  between  it  and  the  shore,  just  as  at  the  beginning  of 
the  previous  winter. 

In  classifying  the  forms  observed  on  the  Great  Lakes  by 
Desor,  Gilbert,  Russell,  and  others,  with  those  observed  on 
tidal  shores  by  Cornish  and  Wheeler,  and  giving  the  English 
names  low  and  ball  to  the  entire  series,  I  have  proceeded  on  the 
assumption  that  the  two  forms  are  similar  in  character  and 
identical  in  origin,  such  differences  as  are  noted  being  due  to 
the  changing  water  level  in  the  case  of  the  marine  type.  I  must 
state,  however,  that  this  procedure  is  not  based  on  any  careful 


RIPPLE   MARKS  489 

comparison  of  these  forms  as  developed  in  lakes  and  in  the 
ocean,  and  my  classification  is  accordingly  to  be  accepted  with 
due  reservation.  While  I  have  examined  fairly  good  lows  and 
balls  along  the  sandy  beach  at  Cape  Henry,  Virginia,  and  else- 
where on  the  Atlantic  shoreline,  I  have  not  seen  those  of  the 
Great  Lakes ;  nor  have  I  made'  sufficient  study  of  the  examples 
observed  to  add  anything  of  value  to  the  discussion  of  their 
origin. 

RIPPLE   MARKS 

The  accumulation  of  sand  and  finer  debris  in  parallel  ridges 
and  troughs  somewhat  resembling  water  waves  in  form,  though 
not  at  all  in  origin  or  method  of  formation,  was  long  ago  recog- 
nized as  a  normal  product  of  wave  and  current  action.  Under 
various  names,  such  as  "  current  mark,"  "  wave  mark,"  "  ripple 
drift,"  "  current  drift,"  and  "  friction  markings,"  the  phenome- 
non now  generally  known  as  ripple  mark  has  repeatedly  been 
described.  Although  not  infrequently  found  on  sandy  beaches, 
ripple  marks  are  perhaps  better  developed  on  tidal  flats  and  over 
the  broad  shallow  bottoms  of  estuaries.  They  are  not  unknown 
on  the  deeper  sea  floor  of  the  offshore  zone,  where  their  occur- 
rence to  a  depth  of  over  600  feet  has  been  demonstrated.  Ripple 
marks  exposed  by  the  falling  tide  may  be  delicately  dissected 
by  rill  marks,  an  interesting  example  of  this  phenomenon  having 
been  described  by  Dodge^^. 

Among  the  earlier  accounts  of  ripple  marks  one  of  the  most  in- 
teresting is  based  on  the  little  known  work  of  an  ingenious  French 
engineer  named  Siau'^.  In  1841  this  investigator  published  a 
brief  note  entitled  "  De  Taction  des  vagues  a  de  grandes  pro- 
fondeurs,"  based  on  observations  of  ripple  marks  in  deep  water 
made  with  the  aid  of  an  ordinary  sounding  apparatus.  While 
examining  ripple  marks,  visible  during  quiet  water,  on  the  bed 
of  a  channel  off  the  west  coast  of  the  Isle  of  Bourbon,  Siau 
noted  that  the  heavier  particles  of  the  sand  tended  to  accumu- 
late in  the  troughs  between  the  ridges,  while  lighter  material 
was  concentrated  along  the  ridge  crests.  Profiting  by  this  dis- 
covery, he  coated  a  sounding  lead  with  tallow,  and  lowered  it  to 
the  sea  floor  where  the  depth  was  too  great  for  direct  visual 
observation.  When  brought  to  the  surface  the  tallow  some- 
times retained,  adhering  to  it,  only  heavy  particles  of  sand,  in 


490 


MINOR  SHORE  FORMS 


Plate  LIX. 


Photo  by  G.  K.  Gilbert,  U.  S.  G.  S. 

Sandstone  slab  showing  fossil  oscillation  ripples.     A  later,  smaller  series  of 
oscillation  ripples  had  begun  to  form  in  the  troughs  of  the  main  series. 


RIPPLE  MARI^ 


491 


which  the  surface  of  the  tallow  had  convex  form,  showing  that 
it  had  been  pressed  down  into  the  trough  between  two  ripples. 
In  other  cases  the  tallow  was  coated  with  lighter  particles  only 
and  had  a  concave  form,  as  a  result  of  having  been  pressed 
down  upon  a  ripple  crest.  At  great  depths,  where  the  ripples 
were  more  closely  spaced,  two  parallel  bands  of  materials,  differ- 
ing in  specific  gravity,  would  be  impressed  upon  the  tallow  at 
the  same  time,  the  heavier  material  coating  a  convex  ridge  and 
the  lighter  a  concave  depression  in  the  tallow.  By  this  ingenious 
device  Siau  was  able  to  prove  the  existence  of  ripple  marks  at  a 
depth  of  617  feet. 

The  ripples  described  by  Siau  were  believed  by  him  to  be 
due  to  the  back-and-forth  currents,  which,  as  we  have  already 
seen,  are  produced  on  a  sea-bottom  by  oscillatory  waves.  Such 
ripple  marks  are  called  ''  oscillation  ripples,"  and  are  char- 
acterized by  symmetry  of  crests,   neither  slope  being  steeper 


Fig.  146.  —  Oscillation  ripples. 

than  the  other,  since  the  ridges  are  ])uilt  up  by  currents  which 
operate  from  either  side  with  approximately  equal  force.  The 
crests  are  sharp  and  narrow  as  compared  with  the  more  broadly 
rounded  intervening  trough  (Fig.  146).  Do  la  Beche^^  in  his 
Geological  Observer  describes  another  type  of  ripple  mark  pro- 
duced by  the  action  of  a  current  flowing  steadily  in  one  direc- 
tion over  a  bed  of  sand.  These  "  current  ripples "  have  a 
long,  gentle  slope  toward  the  direction  from  which  the  current 
comes,  and  a  shorter,  steeper  slope  on  the  lee  side.  Sand  grains 
removed  from  the  gentle  slope  are  carried  to  the  crest  and 
dropped  down  the  steeper  slope,  causing  the  ripples  to  migrate 
slowly  with  the  current,  much  as  sand  dunes  migrate  with  the 
wind.  The  asymmetry  of  profile  of  the  current  ripple  is  shown 
by  Figure  147,  and  is  apparent  in  Plates  LX  and  LXI  Barrell^^  and 
others  restrict  the  term  "  ripple  mark  "  to  oscillation  ripples. 


492 


MINOR  SHORE   FORMS 


RIPPLE  MARKS  493 

and  employ  the  term  "  current  mark  "  for  the  asymmetrical 
type.  This  usage  has  much  to  commend  it,  but  is  open  to  sev- 
eral objections.  The  fact  that  "  current  mark  "  is  produced  by 
water  currents  might  lead  to  the  inference  that  "  ripple  mark  " 
is  produced  by  water  ripples,  which  is  not  at  all  the  case. 
Ordinary   waves    rather    than   true   ripples   commonly   produce 


Fig.  147.  —  Current  ripples. 

oscillation  ripple  marks.  There  are,  moreover,  other  markings 
produced  by  currents,  as  will  appear  on  a  later  page.  The  term 
"  ripple  mark  "  is  so  firmly  established  in  the  literature  to  in- 
clude both  the  symmetrical  and  asymmetrical  types  that  it  seems 
wisest  to  follow  this  usage,  prefixing  the  words  "  oscillation  " 
and  "  current  "  to  make  clear  the  necessary  distinction. 

Sorby^^  gave  a  very  good  description  of  current  ripples  in 
The  Geologist  for  1859,  but  failed  to  recognize  the  existence  of 
wave-formed  oscillation  ripples,  although  he  noted,  and  even 
pressed  too  closely,  the  analogy  between  true  waves  and  ripple 
mark.  For  many  years  current-formed  ripples  were  the  only 
type  recognized  in  most  textbooks.  Gilbert's  in  1875  described 
Ijriefly  what  appear  to  have  been  oscillation  ripples,  but  explained 
them  as  the  product  of  running  water  thrown  into  vibration  by 
friction  on  the  bottom,  a  theory  apparently  similar  to  the  "  in- 
termittent friction  "  theory  of  de  Candolle,  described  below. 
In  1882,  in  opposition  to  the  general  view,  Hunt^^  claimed  that 
as  a  rule  ripple  marks  are  the  product  of  oscillatory  wave  action, 
and  supported  his  claim  with  observations  based  on  the  arti- 
ficial production  of  ripple  marks,  as  well  as  with  numerous 
citations  of  naturally  formed  ripples.  He  was  evidently  un- 
aware of  the  fact  that  Siau  had  supported  the  same  theory  some 
40  years  earlier,  and  in  a  later  paper^''  erroneously  credited 
Forel    with    priority  in  the  recognition  of  oscillation  ripples. 


494  MINOR  SHORE  FORMS 

Hunt  incidentally  describes  oscillation  ripples  in  his  paper  "  On 
the  Action  of  Waves  on  Sea-Beaches  and  Sea-Bottoms^^";  he 
also  discusses  the  nomenclature  of  ripple  marks  at  much  length 
in  a  paper  published  in  1904^'-,  and  elsewhere  quotes  Lieu- 
tenant Damant,  R.N.,  as  having  observed  ripple  marks  while 
diving  at  depths  of  60  and  70  feet^^. 

In  1883,  the  year  following  the  publication  of  Hunt's  earliest 
paper  quoted  above,  there  appeared  three  important  essays  on 
ripple  marks:  one  by  de  Candolle  on  "  Rides  Formees  a  la 
Surface  du  Sable  Depose  au  Fond  de  I'Eau  et  autres  Phenomenes 
Analogues/';  another  by  Forel  on  "  Les  Rides  de  Fond  Etudiees 
dans  le  Lac  Leman  ";  and  a  third  by  Darwin  "  On  the  Forma- 
tion of  Ripple  Mark  in  Sand."  De  Candolle^^  produced  ripple 
marks  artificially  by  experimenting  not  only  with  sand  and  vari- 
ous substances  in  powdered  form  covered  by  water,  but  also 
with  liquids  of  varjang  viscosity  covered  with  water  and  other 
liquids.  Regarding  sand  or  powder  when  mixed  with  water  as 
a  viscous  substance,  he  concluded '  from  his  experiments  that 
"  When  viscous  material  in  contact  with  a  fluid  less  viscous  than 
itself  is  subjected  to  oscillatory  or  intermittent  friction,  result- 
ing either  from  a  movement  of  the  covering  fluid  or  from  a 
movement  of  the  viscous  mass  itself  with  respect  to  the  covering 
fluid,  (1)  the  surface  of  the  viscous  substance  is  ridged  perpen- 
dicularly to  the  direction  of  friction,  and  (2)  the  interval  between 
the  ridges  is  directly  proportional  to  the  amplitude  of  the  friction- 
producing  movement."  That  ripple  marks  depend  on  simple 
friction  alone,  and  not  on  any  change  of  level  in  the  covering 
liquid  such  as  occurs  during  wave  action,  de  Candolle  proved 
by  an  experiment  with  a  rotating  disc  submerged  in  a  tank  of 
water.  After  submerging  the  disc  and  mixing  an  insoluble 
powder  in  the  water,  the  apparatus  was  left  until  the  powder 
settled  on  the  disc  and  floor  of  the  tank  as  an  even  film,  and  the 
water  came  to  rest.  An  oscillatory  rotary  movement  then  ap- 
plied to  the  disc  caused  radiating  ripples  to  form  upon  it,  while 
no  ripples  formed  on  the  stationary  bottom,  and  the  surface  of 
the  water  remained  quiescent.  The  author  concludes  that  the 
formation  of  ripples  in  sand,  whether  under  currents  of  air  or 
under  water  currents,  is  identical  in  origin  with  the  formation 
of  water  ripples  under  moving  air.  If  the  current  moves  always 
in  one  direction  we  have  intermittent  friction  due  to  varying 


RIPPLE   MARKS 


495 


496  MINOR   SHORE   FORMS 

velocities.  Otherwise  we  have  oscillatory  friction  due  to  alter- 
nating change  of  direction.  Current  ripples  result  from  the 
first  type  of  friction,  oscillation  ripples  from  the  second. 

ForeP^  in  his  excellent  essay  on  "  Les  Rides  de  Fond  Etu- 
diees  dans  le  Lac  Leman  "  sets  forth  the  mature  results  of  studies 
which  had  been  briefly  mentioned  by  him  in  three  communica- 
tions^^ of  earlier  date.  Abandoning  his  first  theory,  that  the 
formation  of  ripple  marks  was  dependent  in  part  upon  the 
vertical  pressure  of  water  waves  upon  the  bottom^^  Forel  reached 
the  following  important  conclusions  as  the  result  of  many  care- 
ful observations  and  experiments:  (1)  Current  ripples  are  asym- 
metrical and  migrate  with  the  current  like  ordinary  sand  dunes, 
whereas  oscillation  ripples  are  stationary  and  symmetrical.  (2) 
Each  oscillation  ripple  is  really  a  composite  of  two  current 
ripples  resulting  from  the  action  of  two  currents  moving  alter- 
nately in  opposite  directions,  each  current  attempting  to  form 
the  ridge  into  a  current  ripple  migrating  with  it,  but  being  de- 
feated when  the  return  current  tries  with  equal  force  to  shape  the 
ridge  into  a  current  ripple  directed  in  the  opposite  sense.  (3) 
The  length  of  the  water  body  has  no  direct  effect  on  the  spacing 
of  the  ripples.  (4)  Other  things  equal,  the  ripples  are  more 
closely  spaced  with  increasing  depth.  (5)  At  a  given  depth, 
and  with  other  conditions  uniform,  the  ripples  are  more  widely 
spaced  with  increase  in  coarseness  of  sand  grains.  (6)  Ripples 
once  formed  do  not  experience  a  change  in  spacing  as  a  result 
of  diminishing  amplitude  of  oscillation  of  the  water,  although  the 
original  spacing  does  depend  upon  the  amplitude  of  oscillation, 
as  pointed  out  by  de  Candolle.  (7)  For  any  given  coarseness 
of  sand  grains  there  is  a  certain  mean  velocity  of  the  oscillating 
currents  which  will  produce  ripples:  lower  velocities  will  fail  to 
move  the  sand  grains,  and  hence  cannot  build  ripples,  while 
higher  velocities  agitate  the  whole  mass  of  sand  so  violently 
that  no  ripples  can  form.  (8)  The  formation  of  ripples  is  ini- 
tiated by  some  obstacle  or  inequality  on  the  surface  of  the  sand, 
behind  which  sand  grains  accumulate  in  the  eddy  caused  by  its 
presence:  this  leaves  a  furrow  on  either  side  of  the  initial  ridge, 
due  to  the  abstraction  of  sand  accumulated  in  the  ridge;  and 
these  furrows  in  their  turn  cause  additional  ridges  to  develop  on 
their  outer  margins,  and  so  on.  (9)  In  a  given  locality,  ripple 
marks  almost  always  form  with  the  same  spacing,  regardless  of 


RIPPLE   MARKS 


497 


the  varying  intensity  of  winds  and  waves  affecting  the  water 
body;  this  is  in  consequence  of  laws  7  and  6  stated  above. 
(10)  The  depth  at  which  ripple  marks  may  form  is  hmited  by 
the  depth  to  which  wave  action  may  extend  with  sufficient 
energy  to  move  the  bottom  sands;  hence  it  depends  on  the  size 
of  the  waves,  and  therefore  in  part  indirectly  on  the  size  of  the 
water  body:  in  the  Rhone  the  limiting  depth  is  a  few  decimeters; 
in  Lake  Geneva  some  ten  meters;  and  in  the  ocean  from  20  to 
188  meters,  according  to  Lyell  and  Siau.  Forel  revised  de  Can- 
dolle's  law  regarding  the  relation  of  ripple  spacing  to  amplitude 
of  the  friction-producing  movement  to  read:  "  The  breadth  of 
the  ripples,  or  the  distance  from  one  crest  to  another,  is  the 
length  of  the  path  followed  during  a  single  oscillation  by  a  grain 
of  sand  freely  transported  by  the  water."  The  length  of  this 
p£,th  varies  directly  as  the  horizontal  amplitude  of  the  oscilla- 
tory movement  of  the  water,  directly  as  the  velocity  of  that 
movement,  inversely  as  the  density  of  the  sand,  and  inversely 
as  the  size  of  the  sand  grains. 

Darwin's   paper'^^   "  On   the   Formation   of    Ripple   Mark   in 
Sand  "  is  especially  noteworthy  for  its  careful  analysis  of  the 


Fig.  1 18.  —  Vortices  involved  in  the  formation  of  cuiTent  ripple  mark. 

vortices  which  are  so  important  a  factor  in  the  construction  of 
the  ripples.  When  symmetrical  oscillation  ripples  were  sub- 
jected to  the  action  of  a  steady  current,  Darwin  noticed  that 
not  only  did  sand  grains  migrate  up  the  weather  slope  of  each 
ripple  with  the  current,  but  that  they  also  ascended  the  lee 
slopes,  apparently  against  the  current.  This  proved  conclu- 
sively the  existence  of  such  vortices  as  are  represented  in  Figure 
148.  Darwin  then  proceeded  to  study  the  vortices  by  watching 
the  movements  of  a  drop  of  ink  released  from  the  end  of  a  fine 
glass  tube  at  that  point  in  the  water  where  the  action  was  to  be 
observed.  In  this  manner  the  vortices  associated  with  the  alter- 
nating currents  which  produce  oscillation  ripples  were  analyzed 


498  MINOR  SHORE  FORMS 

with  a  high  degree  of  precision,  and  much  Hght  was  thrown  upon 
the  metliod  of  ripple  growth.  Darwin  concluded  that  "  the 
formation  of  irregular  ripple  marks  or  dunes  (current  ripples)  by 
a  current  is  due  to  the  vortex  which  exists  on  the  lee  side  of  any 
superficial  inequality  of  the  bottom;  the  direct  current  carries 
the  sand  up  the  weather  slope  and  the  vortex  up  the  lee  slope. 
Thus,  any  existing  inequalities  are  increased  and  the  surface  of 
sand  becomes  mottled  over  with  irregular  dunes."  The  in- 
termittent friction  to  which  de  Candolle  appealed  is  not  essen- 
tial in  this  explanation  of  current  ripples.  Oscillation  ripples 
of  regular  pattern  are  changed  by  a  continuous  current  into 
regularly  spaced  current  ripples;  but  a  uniform  current  cannot 
of  itself  initiate  regularly  spaced  ripple  mark.  "  Regular  ripple 
mark  (oscillation  ripples)  is  formed  by  water  which  oscillates 
relatively  to  the  bottom.  A  pair  of  vortices,  or  in  some  cases 
four  vortices,  are  established  in  the  water;  each  set  of  vortices 
corresponds  to  a  single  ripple  crest."  Forel's  conception  of  an 
oscillation  ripple  as  a  composite  of  two  dunes  (current  ripples) 
formed  alternately  by  oscillating  currents  is  regarded  as  correct; 
but  his  law  for  the  relation  of  ripple  spacing  to  amplitude  of 
oscillation  is  believed  to  require  some  modification. 

Further  studies  of  ripple-forming  vortices  were  made  by  Mrs. 
Hertha  Ayrton^^  the  results  of  which  were  not  published  until 
1910.  With  the  aid  of  well-soaked  grains  of  ground  black 
pepper,  or  of  particles  of  potassium  permanganate  dissolving 
and  coloring  the  water  while  the  latter  was  in  oscillation,  she 
observed  the  formation  of  vortices  and  endeavored  to  explain 
the  mechanics  of  their  growth.  Although  she  expressed  dis- 
agreement with  the  conclusions  of  Darwin  and  others  on  certain 
points,  most  of  her  results  afford  essential  confirmation  of  their 
main  contentions.  Some  doubt  must  attach  to  certain  of  her 
deductions,  such  as  one  to  the  effect  that  no  ripple-forming 
vortex  occurs  in  the  lee  of  an  obstacle  over  which  a  steady  cur- 
rent is  passing  and  hence  "  a  steady  current  is  unable  either  to 
generate  or  to  maintain  ripple-mark." 

The  British  Association  Reports  for  the  years  1889,  1890,  and 
1891,  contain  three  papers  by  Reynolds^"  on  the  action  of 
waves  and  currents  in  model  estuaries,  in  which  are  some  valu- 
able observations  regarding  what  may  well  be  termed  giant 
tidal  ripples.     While  experimenting  with  artificial  tidal  currents 


RIPPLE  MARKS  499 

Plate  LXII. 


Photo  by  G.  K.  Gilbert. 
Giant  current  ripples  near  Annisquam,  Massachusetts,  showing  irregular 
pattern  due  to  interference  of  wave  and  tidal  currents. 


500  MINOR  SHORE  FORMS 

Reynolds  discovered  that  current  ripples  were  formed  in  the 
model  estuaries.  By  making  due  allowance  for  the  difference 
in  size  between  the  model  estuaries  and  those  in  nature,  he 
concluded  that  real  tidal  currents  ought  to  produce  very  large 
current  ripples,  possibly  7  or  8  feet  in  height  and  80  to  100  feet 
aparf^  Some  years  later  Vaughan  Cornish''^  discovered  nat- 
ural tidal  ripples  or  "  sand  waves  "  of  the  same  type  as  those 
produced  artificially  by  Reynolds,  having  a  height  of  2  feet  and 
an  average  distance  of  more  than  37  feet  from  crest  to  crest. 
In  two  later  papers^^  Cornish  describes  giant  tidal  ripples  more 
fully,  and  illustrates  their  essential  features  with  a  large  series 
of  beautiful  photographs.  Some  of  these  ripples  have  a  height 
of  nearly  3  feet  above  the  intervening  troughs,  and  a  distance 
between  crests  of  from  66  to  88  feet  in  extreme  cases.  The 
giant  ripples  are  often  covered  with  ordinary  ripple  mark,  and 
while  Cornish  recognized  that  the  larger  forms  were  produced 
by  the  continuous  steady  flow  of  tidal  currents,  he  was  at  first 
inclined  to  invoke  pulsatory  currents  in  order  to  explain  the 
smaller  ripple  mark^^.  This  theory  seems  to  be  a  survival  of 
de  Candolle's  erroneous  idea  that  "  intermittent  friction  "  was 
essential  to  the  production  of  current  ripples,  and  is  practically 
abandoned  by  Cornish  in  his  more  recently  published  book  on 
"  Waves  of  Sand  and  Snow^^."  Gilmore^^  describes  tidal  ripples 
on  the  Goodwin  Sands  having  a  height  of  "  two  or  three  feet," 
and  Kindle*'^  reports  "  mammoth  tidal  ripples  "  in  estuaries  of 
the  Bay  of  Fundy  varying  in  length  from  2  feet  up  to  15  or  20 
feet,  and  in  height  from  6  inches  to  nearly  2  feet.  Gilbert^^ 
measured  examples  near  Annisquam,  Massachusetts,  which  were 
15  feet  in  length  and  15  inches  high,  Plate  LXII.  River  currents 
as  well  as  tidal  currents  are  capable  of  forming  giant  ripples,  and 
Kindle^^  describes  examples  formed  on  a  broad  sandbar  in  the 
Ottawa  River  at  time  of  flood  which  measured  30  to  45  feet  in 
length  and  from  1  to  2  feet  in  height.  The  same  author  quotes 
Pierce  as  authority  for  the  existence  in  the  San  Juan  River  in 
Utah  of  examples  rising  3  feet  above  the  adjacent  troughs.  Un- 
fortunately Pierce^"  improperly  applies  the  term  "  sand  wave  "  to 
the  water  wave  formed  at  the  surface  of  a  current  passing  over 
true  sand  waves  or  giant  ripples.  It  is  not  clear  that  Pierce 
either  saw  or  measured  the  giant  sand  ripples,  supposed  by  him  to 
have  caused  the  surface  water  waves  to  which  his  figures  apply. 


RIPPLE   MARKS  501 

It  should  be  noted  that  all  of  the  giant  ripples  referred  to 
above  belong  to  the  unsynimetrical  type;  they  are  true  current 
ripples.  So  far  as  I  am  aware  no  giant  oscillation  ripples  have 
ever  been  observed  along  modern  shores.  It  may  be  doubted 
whether  tidal  currents  could  form  symmetrical  ripples,  notwith- 
standing Reynold's  suggestion  to  the  contrary^^  The  flow  and 
ebb  of  the  tide  constitutes  an  oscillating  current,  it  is  true;  but 
the  currents  are  often  of  unequal  force.  Where  equally  strong, 
each  current  persists  long  enough  to  remodel  the  ridges  formed 
by  the  preceding  current,  giving  them  an  asymmetrical  form 
appropriate  to  the  current  operating  last.  On  the  other  hand, 
Gilbert^^  has  described  structures  in  the  Medina  sandstone 
formation  of  New  York  which  he  believes  to  be  giant  ripples 
of  the  symmetrical  type  formed  by  oscillating  currents  due  to 
wave  action.  In  dimensions  these  ridges  were  similar  to  the 
average  examples  of  tidal  ripples  described  by  Cornish,  having 
a  height  of  from  6  inches  to  3  feet  and  a  distance  from  crest  to 
crest  of  10  to  30  feet;  but  their  nearly  symmetrical  form  did 
not  suggest  a  similar  origin.  Gilbert  reached  the  tentative  con- 
clusion that  they  were  formed  by  waves  60  feet  high  in  deep 
water  of  a  broad  ocean.  This  conclusion  was  criticized  by 
FairchikP^,  who  advanced  convincing  arguments  in  support  of 
the  opinion  that  the  forms  in  question  were  beach  structures, 
possibly  successive  beach  ridges  built  on  the  strand.  Branner^* 
suggested  that  they  might  represent  fossil  beach  cusps  seen  in 
cross-section. 

Some  interesting  experiments  on  the  relation  of  current  ve- 
locity to  ripple-mark  formation  were  made  by  Owens'^^,  who 
published  his  results  in  1908.  He  found  that  currents  from  0.85 
to  2.5  feet  per  second  produced  or  maintained  a  rippled  surface 
on  sand;  but  that  a  velocity  of  2.5  feet  per  second  and  above 
swept  the  surface  free  of  ripples. 

In  1911  A.  P.  Brown^"  published  a  paper  entitled  "  The 
Formation  of  Ripple-Marks,  Tracks  and  Trails  "  in  which  he 
endeavored  to  show  that  asymmetrical  ripples  (current  ripples) 
were  formed  by  deposition,  whereas  symmetrical  ripples  (oscil- 
lation ripples)  resulted  from  the  erosion  of  a  formerly  smooth 
bottom  consequent  upon  the  rippling  of  overlying  w^ter  by 
wind  action.  His  conclusions  do  not  appear  to  be  supported 
by  a  sufficient  body  of  convincing  evidence,  and  are  opposed 


502  MINOR  SHORE  FORMS 

by  theoretical  considerations  and  by  the  great  body  of  experi- 
mental data  already  referred  to  on  previous  pages.  Unfortu- 
nately, in  presenting  his  theory  Brown  does  not  consider  the 
important  results  obtained  in  the  many  previous  investigations 
of  ripple  marks. 

A  similar  criticism  must  be  urged  against  the  work  of  Epry'^ 
who  in  1912  published  a  paper  on  "  Les  Ripple-Marks  "  in  the 
Annales  de  I'lnstitut  Oceanographique.  Epry  states  that  no 
one  before  him  has  been  able  accurately  to  determine  the  causes 
of  ripple  marks  and  that  no  previous  theory  of  their  origin  is 
satisfactory.  He  fails,  however,  to  show  wherein  earlier  the- 
ories are  defective  and  from  his  essay  it  does  not  appear  that  he 
was  acquainted  with  the  various  publications  cited  above. 
Current  ripples  and  oscillation  ripples  are  not  distinguished  by 
him,  and  a  highly  specialized  theory  of  origin,  impossible  of 
application  to  the  majority  of  ripple  surfaces,  is  developed.  It 
is  not  necessary  to  criticize  Epry's  theory  in  detail,  but  a  gen- 
eral idea  of  its  essential  nature  may  be  gathered  from  the  fact 
that  it  involves  the  remarkable  assumption  that  ripples  are 
formed  where  an  ebbing  tidal  current  returning  from  the  shore 
is  cut  transversely  by  another  current  deflected  along  a  depres- 
sion in  the  sea  floor,  and  that  the  ripples  are  aligned  in  -  the 
direction  of  (parallel  to)  the  transverse  current.  No  less  re- 
markable is  Epry's  statement  that  ripple  marks  are  the  work  of 
tides  alone. 

We  have  already  noted  that  current  ripples,  like  sand  dunes, 
normally  migrate  slowly  in  the  direction  of  the  current  which  is 
fashioning  them.  Vaughan  Cornish^^  discovered,  however,  that 
in  shallow  water  when  the  current  attains  a  velocity  of  about 
2.2  feet  per  second,  the  ripples  travel  upstream  or  against 
the  current.  This  observation  was  later  confirmed  by  Owens'^^, 
and  the  phenomenon  is  described  by  Gilbert^''  in  the  following 
words:  "When  the  conditions  are  such  that  the  bed  load  is 
small,  the  bed  is  molded  into  hills,  called  dunes,  which  travel 
downstream.  Their  mode  of  advance  is  like  that  of  eolian 
dunes,  the  current  eroding  their  upstream  faces  and  depositing 
the  eroded  material  on  the  downstream  faces.  With  any  pro- 
gressive change  of  conditions  tending  to  increase  the  load,  the 
dunes  eventually  disappear  and  the  debris  surface  becomes 
smooth.     The  smooth  phase  is  in  turn  succeeded  by  a  second 


RIPPLE   MARKS 


503 


H 


i$5:i 


504  MINOR  SHORE  FORMS     - 

rhythmic  phase,  in  which  a  system  of  hills  travels  upstream. 
These  are  called  anti-dunes,  and  their  movement  is  accomplished 
by  erosion  on  the  downstream  face  and  deposition  on  the  up- 
stream face.  Both  rhythms  of  debris  movement  are  initiated 
by  rhythms  of  water  movement."  Pierce^^  states  that  the 
anti-dune  movement  is  best  seen  "  only  on  heavily  loaded  silt 
streams,"  and  cites  cases  of  the  phenomenon  in  the  San  Juan 
River  in  Utah. 

The  best  recent  essay  on  ripple  marks  is  a  paper  by  Kindle^^ 
entitled  "  Recent  and  Fossil  Ripple  Mark,"  pubHshed  in  1917. 
This  author  presents  an  excellent  summary  of  his  own  exten- 
sive observations,  distinguishes  the  different  types  of  ripple 
marks  and  their  methods  of  origin,  and  gives  many  references 
to  the  work  of  others.  The  abundant  illustrations  contain 
some  of  the  best  views  of  ripple  marks  ever  published.  Kindle 
studied  different  types  of  ripples  by  means  of  plaster  casts,  some 
of  which  were  secured  at  depths  ranging  up  to  27  feet  by  means 
of  a  specially  devised  apparatus.  Siau's  experiments  were  also 
imitated  by  lowering  to  the  bottom,  at  any  depth,  a  rectangular 
plate  of  sheet  iron  or  zinc,  the  under  surface  of  which  had  been 
coated  with  vaseline.  Where  ripple  marks  occurred,  parallel 
hnes  of  sand  adhering  to  the  vaseline  indicated  the  position 
and  spacing  of  the  ripple  crests.  On  the  basis  of  his  studies 
Kindle  concludes  that  the  length  of  asjnnmetrical  or  current 
ripples  varies  with  the  velocity  of  the  current,  with  the  volume 
of  sediment  in  suspension,  and  possibly  also  with  depth.  "  A 
strong  current  carrying  a  maximum  load  of  sand  probably  forms 
ripple  mark  of  large  amplitude  (length)  where  a  slightly  loaded 
current  having  the  same  velocity  would  leave  no  ripple  mark." 
The  author  is  less  certain  about  the  factors  controlling  symmet- 
rical or  oscillation  ripples,  but  thinks  coarseness  of  sand,  depth 
of  water,  and  length  of  the  water  waves  are  of  chief  importance. 
In  studying  Kindle's  valuable  paper  the  reader  must  guard 
against  misapprehension  arising  from  his  use  of  the  term  "  am- 
plitude "  to  denote  the  le7igth  of  both  sand  waves  and  water 
waves. 

Some  of  Kindle's  conclusions  must  be  regarded  as  open  to 
question.  This  is  particularly  true  of  the  following  general- 
izations: "  On  the  shores  of  lakes  where  ripple  mark  is  due 
entirely  to  wave  action  it  always  runs  parallel  with  the  coast- 


RIPPLE   MARKS  505 

line.  Ripple  mark  along  the  sea  coast  is  generally  the  work  of 
tidal  currents  which  follow  the  shorehne.  These  cui'rent-made 
ripple  marks  consequently  trend  at  right  angles  to  the  coast- 
line. Lake  shore  and  sea  shore  ripple  mark  are  thus  differently 
oriented  with  respect  to  their  adjacent  shorelines,  the  former 
trending  with  the  shoreline,  the  latter  at  right  angles  to  it*'  "; 
"the  abundance  of  the  wave-made  type  of  ripple  mark  in  a 
sandstone  formation  and  the  absence  of  the  asymmetrical  type 
would  indicate  its  formation  under  lacustrine  conditions.  The 
great  predominance  on  the  other  hand  of  the  asymmetrical 
type  of  ripple  mark  would  as  certainly  suggest  the  work  of 
tidal  current  action  and  marine  conditions^."  My  own  ob- 
servations of  ripple  marks  do  not  tend  to  support  the  conclu- 
sions expressed  in  these  quotations.  While  it  is  true  that  wave 
refraction  often  brings  about  a  more  or  less  perfect  parallelism 
between  wave  crests  and  the  shoreline  in  the  immediate  vicinity 
of  the  latter,  the  parallelism  is,  on  the  other  hand,  often  far 
from  perfect;  and  a  few  feet  from  the  shore  the  waves  not  in- 
frequently trend  at  large  angles  to  the  shore.  I  have,  on  a  number 
of  occasions,  observed  ripples  on  the  bottoms  of  ponds  and  lakes 
which  were,  like  the  waves  which  formed  them,  not  parallel  to 
the  shoreline  even  when  but  a  few  feet  distant  from  it.  The 
supposed  restriction  of  oscillation  ripples  to  lacustrine  deposits 
seems  equally  doubtful.  Some  of  the  best  oscillation  ripples  I 
have  ever  seen  were  formed  on  tidal  fiats,  bordering  the  Long 
Island  shore,  by  wave  action  when  shallow  water  covered  the 
flats  at  high  tide.  Other  good  examples  may  frequently  be  seen 
in  shallow  ponds  and  abandoned  channels  on  river  flood  plains. 
Kindle's  discriminations  between  marine  and  lacustrine  deposits 
(see  pp.  48  to  51  of  his  essay),  and  between  lacustrine  and  fluvia- 
tile  deposits  (pp.  52  and  53),  on  the  basis  of  the  type  of  the  con- 
tained ripple  marks,  must  therefore  be  accepted  with  caution, 
just  as  truly  as  must  his  deductions  regarding  the  direction  of 
ancient  shorelines  based  on  the  orientation  of  fossil  ripple  marks. 
Even  where  a  geological  formation  contains  ripple  marks  ex- 
hibiting a  remarkable  uniformity  of  orientation  over  wide  areas, 
as  in  a  case  described  by  Hyde^  in  a  valuable  paper  published 
a  few  years  ago,  and  where  the  existence  of  some  definite  control 
of  ripple  direction  is  clearly  demonstrated,  there  may  still  be 
room  for  a  variety  of  interpretations  as  to  the  position  of  former 


506 


MINOR  SHORE  FORMS 


'^^f^-^j'^^^'^     ""-"^        '      '    '    ^' 


RIPPLE  MARKS  507 

shorelines.  An  interesting  attempt  to  deduce  paleogeographic 
conditions  from  a  discriminating  study  of  large  fossil  current 
ripples  will  he  found  in  a  recent  paper  by  Bucher^". 

The  simultaneous  action  of  continuing  currents  and  oscilla- 
tory wave  motion,   as  well  as  the  action  of  intersecting  cur- 
rents or  intersecting  systems  of  waves,  give  rise  to  a  variety  of 
pecuHar  ripple  forms.     Thus  oscillation  ripples  may  be  made 
slightly  asymmetrical  by  a  feeble  current,   or  faint  oscillation 
ripples  may  be  superposed  on  strongly  developed  current  rip- 
ples.    Strong  oscillatory  wave  action  or  secondary  current  action 
may  give  to  a  series  of  current  ripples  the  pecuhar  pattern  shown 
in  Plate  LXIV,  if  the  waves  or  current  advance  obliquely  over 
the  earlier  formed  current  ripples.     "  Interference  ripple  mark  " 
(Plate  LXV)  results  when  two  sets  of  symmetrical  ripples  are 
formed  by  two  systems  of  waves  crossing  nearly  at  right  angles. 
The  cell-like  pattern  of  some  interference  ripple  marks  led  Hitch- 
cock to  regard  them  as  "  tadpole  nests."     Examples  of  these  and 
other  abnormal  ripple  types  are  described  and  figured  by  Kindle. 
Ripple  marks  have  repeatedly  been  discussed  in  connection 
with  the  interpretation  of  fossil  ripples  found  in  sedimentary 
rocks.     We  need  mention  but  a  few  of  these  discussions  in  the 
present  connection.     As  early  as  1831  Scrope^^  described  fossil 
ripple  marks  found  on  slabs  of  marble,  and  explained  them  as 
due  to  the  oscillatory  movements  of  shallow  water.     Darwin^^, 
starting  from  the  very  questionable  assumption  that  a  great 
ebb  and  flow  of  the  tide  is  essential  to  the  formation  of  numerous 
ripples,  concluded  that  the  presence  of  a  large  number  of  ripple 
marks  in  a  geological  formation  indicated  with  a  considerable 
degree  of  probability  that  the  tides  of  early  times  rose  higher 
than  those  of  today.     Van  Hise^^  figures  and  describes  one  type 
of   oscillation   ripples,    and   emphasizes   their   value   as   criteria 
for  determining  the  original  attitude  of  steeply  inclined  strata. 
Gilbert^"  suggested  the  possibility  of  an  analogy  between  ripple 
marks  and  vibrations  in  elastic  bodies,  basing  the  suggestion 
on  observations  of  fossil  ripple  marks  in  the  Jurassic  limestone 
and  Triassic  sandstones  of  Utah. 

Spurr^i  shows  that  where  continuous  deposition  takes  place 
from  a  current  which  constantly  maintains  asymmetrical  ripples 
on  the  surface  over  which  it  flows,  the  forward  movement  of 
the  ripples  combines  with  the  deposition  of  heavier  and  larger 


508 


MINOR  SHORE  FORMS 


RIPPLE   MARKS  509 

fragments  in  the  troughs  and  Kghter  particles  on  the  crests,  to 
give  a  peculiar  type  of  false  bedding  in  the  resulting  formation. 
Jagger^-  criticized  Spurr's  conclusions  on  the  ground  that  his 
own  experiments  and  observations  indicated  that  ripple  marks 
could  not  be  produced  in  heterogeneous  material;  but  Spurr^^ 
met  the  criticisms  with  a  fuller  discussion  of  the  matter  in  which 
his  original  contention  is  well  sustained  A  short  time  pre- 
viously Sorby^  had  described  a  somewhat  similar  phenomenon 
in  a  paper  printed  almost  exactly  half  a  century  after  the  pub- 
Ucation  of  his  first  account  of  ripple  marks,  already  cited.  From 
an  examination  of  the  "  ripple  drift"  type  of  false  bedding  in 
rocks  Sorby  believed  one  could  "  ascertain  with  approximate 
accuracy,  not  only  the  direction  of  the  current  and  its  velocity 
in  feet  per  second,  but  also  the  rate  of  deposition  in  fractions  of 
an  inch  per  minute^^." 

The  finding  of  ripple-marked  limestone  has  been  the  occasion 
of  two  lines  of  reasoning  regarding  the  origin  of  the  rock,  both 
based  on  the  assumption  that  ripple  marks  cannot  be  formed  in 
deep  water.  According  to  one  argument,  the  ridges  and  troughs 
are  not  true  ripple  marks,  since  limestones  are  necessarily  formed 
in  deep  water;  the  other  argument  holds  such  limestones  to 
be  necessarily  of  shallow  water  origin,  because  the  ridges  and 
troughs  are  true  ripple  marks.  An  example  of  the  former  argu- 
ment may  be  found  in  Locke's  early  report^^  on  "  waved  strata  " 
of  Ordovician  limestone  in  southwestern  Ohio;  while  Foerste^^ 
presents  the  second  point  of  view  in  discussing  the  origin  of 
Ordovician  and  Silurian  beds  in  this  same  general  region.  The 
frequent  occurrence  of  unusually  large  ripple  marks  in  lime- 
stone has  been  noted  by  Gilbert^^  Moore  and  Hole^^  Cushing^°", 
Miller^oS  Kindle  and  Taylor^o^  Udden^o^  Prosserio^,  and  others,  the 
distance  from  crest  to  crest  of  these  ripples  varying  from  one 
foot  to  two  or  three  feet  in  most  cases,  but  reaching  a  maximum 
of  nearly  six  feet  in  an  example  described  by  Udden.  Wooster^o^ 
Kindle^"^,  and  Udden^'^^  record  the  association  of  ripple  marks  in 
limestone  with  the  remains  of  deep  water  organisms;  while 
Kindle^"*  regards  the  large  size  of  the  ripples  as  independent 
evidence  of  a  considerable  water  depth.  Shannon^°^  found  large 
ripple  marks  in  limestone  associated  with  sun-cracks,  but  does 
not  state  whether  the  ripples  were  of  the  symmetrical  or  asym- 
metrical (current)  type.     The  present  writer  published  in  the 


510  MINOR  SHORE  FORMS 

Journal  of  Geology  for  November,  1916,  a  review  of  the  litera- 
ture on  ripple  marks  under  the  title  ''  Contributions  to  the 
Study  of  Ripplemarks^^","  based  on  studies  made  for  the  present 
work. 

The  writer's  observations  of  beaches  incline  him  to  the  opinion 
that  there  is  comparatively  little  chance  for  the  preservation 
and  incorporation  in  the  geological  record  of  ripple  marks  origi- 
nally formed  on  typical  beaches.  As  we  have  alread}^  seen,  the 
beach  is  a  temporary  and  constantly  changing  deposit,  and 
while  both  oscillation  and  current  ripples  form  on  sandy  beaches, 
their  subsequent  destruction  is  almost  certain,  even  though 
streams  discharging  sediment  upon  the  beach  may  temporarily 
bury  them.  Ripple  marks  formed  on  the  sea-bottom  in  the 
offshore  zone  stand  a  better  chance  for  preservation,  as  also  do 
those  on  tidal  mud  flats  and  sand  flats.  Under  none  of  these 
circumstances,  how^ever,  would  the  opportunities  for  preserva- 
tion seem  so  good  as  on  river  flood  plains  and  deltas.  Here 
ripple  marks  of  both  principal  types  are  readily  formed  in 
shallow  ponds,  lakes,  and  stream  channels,  and  later  deposition 
from  spreading  flood  waters  may  quietly  bury  them  in  places 
secure  from  future  disturbance.  Fossil  ripple  marks  are  there- 
fore not  to  be  regarded  as  an  evidence  of  beach  deposits,  unless 
associated  with  independent  evidence  of  a  much  more  reliable 
character. 

Even  where  fossil  ripple  marks  have  a  marine  origin,  their 
position  furnishes  no  satisfactory  clue  to  the  position  of  the 
former  shoreline.  Both  on  the  beach  and  in  the  offshore  zone 
the  axes  of  the  ripples  may  lie  at  any  angle  to  the  shoreline,  as 
has  been  pointed  out  in  earlier  pages.  Current  ripples  with 
axes  at  variable  angles  to  the  shore  are  verj-  frequently  found 
in  low  depressions  along  the  beach.  Water  from  the  rising  tide 
or  from  storm  waves  entering  such  depressions  at  an}-  low  point, 
flows  through  it  developing  transverse  series  of  asymmetrical 
ridges.  Oscillation  ripples  may  take  any  position  on  the  beach, 
and  one  occasionally  sees  there  a  checkerboard  pattern  of  little 
hollows  and  mounds  representing  two  sets  of  oscillation  ripples 
crossing  each  other  at  right  angles.  In  the  offshore  zone  both 
types  of  ripples  have  been  observed  making  high  angles  with 
the  shoreline. 

Regarding  the  relation  existing  between  size  of  ripple  marks 


RIPPLE   MARKS 


511 


-~dil-t£fJi^^M 


512     •  MINOR  SHORE  FORMS 

and  depth  of  water  in  which  they  were  formed,  theoretical  con- 
siderations based  on  the  nature  of  current  and  wave  action 
would  seem  to  compel  the  following  conclusions:  (1)  Giant 
current  ripples  manifestly  cannot  be  produced  in  extremely 
shallow  water;  but  aside  from  this  narrow  limitation,  both 
large  and  small  current  ripples  may  be  formed  in  either  shallow 
or  deep  water.  (2)  Large  oscillation  ripples  cannot  be  formed 
in  shallow  water,  for  large  oscillatory  waves  are  impossible 
where  the  depth  is  small.  (3)  Both  large  and  small  oscillation 
ripples  may  be  formed  in  deep  water;  whether  the  ripples  will 
be  large  or  small  will  depend  upon  a  number  of  factors,  among 
which  the  length  and  height  of  the  wave  and  the  depth  of  the 
water  are  most  important.  The  fact  that  small  ripples  alone 
are  most  commonly  found  in  sandstones,  while  both  large  and 
small  ripples  occur  in  limestones,  is  in  accordance  with  con- 
clusions (2)  and  (3)  above;  while  the  predominance  of  large 
ripples  in  limestones  might  be  expected  to  follow  from  the  sixth 
law  enunciated  by  Forel:  "  Ripples  once  formed  do  not  experi- 
ence a  change  in  spacing  as  a  result  of  diminishing  amplitude  of 
oscillation  of  the  water."  Large  ripples  once  formed  in  deep 
water  tend  to  remain,  and  so  to  be  preserved  by  burial,  despite 
later  oscillations  which  would  of  themselves  have  produced 
closely  spaced  ripples. 

RILL   MARKS 

The  water  left  in  the  sands  of  the  upper  part  of  the  beach 
after  the  retreat  of  the  tide  or  after  the  dying  down  of  storm 
waves,  often  carves  tiny  drainage  channels  as  it  flows  back  to 
the  sea.  These  miniature  river  systems  are  known  as  rill  marks, 
and  are  not  formed  below  sealevel.  They  may,  however,  be 
formed  on  any  slope  of  fine-grained,  imconsolidated  material 
from  the  upper  portion  of  which  there  is  a  seepage  of  water, 
and  hence  their  presence  in  consolidated  rocks  is  no  proof  that 
the  rocks  in  question  represent  beach  deposits.  As  is  the  case 
with  ripple  marks,  the  probability  of  preservation  is  not  so  great 
when  rill  marks  are  formed  on  beaches  as  when  they  are  formed 
elsewhere. 

The  pattern  of  rill  marks  (Plate  XL VI)  often  resembles  rather 
closely  that  of  branching  plant  stems;  indeed,  so  close  is  the 
resemblance  that  casts  of  rill  marks  found  in  sedimentary  rocks 


RILL  MARKS 


513 
Plate  LXVU. 


'H. 


\,,  ._    . 

Photo  by  E.  xM.  Kindle. 

Plaster  cast  of  swash  marks  left  by  four  successive  waves  on  the  sandy  shore 

of  Lake  Erie. 


514  MINOR  SHORE  FORMS 

have  repeatedly  been  mistaken  for  ancient  plant  remains.  In 
1873  Nathorst^^^  published  a  paper  in  which  he  showed  that  rill 
marks  and  other  markings  on  the  strand  had  been  regarded  by 
many  authors  as  phenomena  of  vegetable  origin.  I  have  not 
seen  this  paper,  but  the  same  idea  is  briefly  presented  in  the  same 
author's  valuable  memoir  entitled  "  Om  spar  af  nagra  evertebre- 
rade  djur  m.  m.  och  deras  paleontologiska  betydelse^^^,"  which 
appeared  eight  years  later.  This  memoir  contains  an  excellent 
l3ibliography  of  papers  treating  mechanical  markings  and  the 
tracks  of  animals  on  the  shore  as  vegetable  remains,  and  gave  rise 
to  a  spirited  controversy  in  which  de  Saporta^^^  Nathorst^^^, 
Gaudry^^^,  Williamson"*',  and  others  took  an  active  part.  Wil- 
liamson made  plaster  casts  of  natural  rill  marks  and  showed  their 
identity  with  many  so-called  fossil  plants.  The  reader  who 
would  follow  this  phase  of  the  subject  further  will  find  addi- 
tional references  to  the  literature  in  the  works  of  the  authors 
just  cited. 

RiU  marks  of  an  unusually  delicate  pattern  have  been  briefly 
described  by  Dodge"''  who  found  them  confined  to  the  seaward 
side  of  previously  formed  ripple  marks  on  Winthrop  Beach, 
Massachusetts.  Jagger"^  produced  artificial  rill  marks,  and  de- 
scribed the  process  of  their  development.  Grabau"^  classes 
with  rill  marks  those  branching  distributaries  of  small  streams 
which  debouch  upon  a  beach  or  other  sandy  or  clayey  plain. 
Rill  marks  of  whatever  type  present  no  difficulties  as  to  their 
origin,  while  in  form  they  are  so  simple  and  unimportant  as  to 
require  no  special  discussion. 


SWASH  MARKS 

When  a  wave  breaks  at  the  foot  of  a  gently  inclined  beach, 
part  of  the  water  glides  up  the  slope  in  a  thin  sheet  known  as 
the  "  swash."  After  the  retreat  of  the  swash  the  greatest 
advance  of  its  irregular  margin  is  often  indicated  by  a  thin, 
wavy  line  of  fine  sand,  mica  scales,  bits  of  seaweed  and  other 
debris,  commonly  referred  to  as  a  "wave  mark"  (Plate  LXVII). 
Since  there  are  a  variety  of  marks  left  on  sand  by  wave  action, 
and  the  present  feature  is  peculiarly  a  product  of  the  swash,  I 
have  given  it  the  name  of  "  swash  mark."  Although  too  deli- 
cate a  feature  to  attract  much  attention  on  the  modern  shore, 


BACKWASH   MARKS 


515 


516 


MINOR     SHORE      FORMS 


Plate  LXIX. 


1 

f 


fT^^  rr-^  h     '  mif  '       tarn     ' 


Phoio  oy  ej.  M.  KiTuile. 

Plaster  cast  of  backwash  marks  (after  Kindle). 


BACKWASH   MARKS  517 

the  swash  mark  is  one  of  the  best  proofs  of  beach  action  usually- 
preserved  in  sedimentary  rocks.  When  found  in  the  fossil  con- 
dition swash  marks  may  throw  light  on  other  buried  shore 
forms  with  which  they  are  associated^^". 


BACKWASH   MARKS 

The  return  flow  of  the  swash  down  the  beach  often  develops  a 
peculiar  criss-cross  ridge  pattern  (Plate  LXVIII)  in  the  sand  re- 
sembling "  the  overlapping  scale-leaves  of  some  Cycadean  stem." 
Williamsoni2i  regarded  similar  ridge  patterns  as  the  product  of 
intersecting  ripple  marks  trenched  by  subsequent  rills.  The 
illustrations  given  by  him  do  not  suggest  such  an  origin,  and  I 
am  inclined  to  regard  the  phenomena  observed  by  him  as  iden- 
tical in  origin  with  the  criss-cross  pattern  which  I  have  observed 
in  process  of  formation  by  the  backwash.  Kindlei22  figures  an 
excellent  example  of  the  phenomenon  under  the  title  "  imbricated 
wave  sculpture"  (Plate  LXIX),  and  ascribes  it  to  ''  the  action  of 
very  small  waves  lapping  and  crossing  each  other  from  opposite 
sides  of  a  miniature  spit."  It  is  a  matter  of  common  observa- 
tion that  two  projecting  lobes  of  the  swash  are  often  directed 
toward  each  other  as  they  rush  up  the  beach  slope,  and  that  the 
return  backwash  from  the  two  meet  at  an  angle  in  their  de- 
scent. The  resultant  crossing  of  currents  would  be  similar  to 
that  described  by  Kindle,  and  might  explain  the  frequent  devel- 
opment of  the  imbricated  pattern  on  beaches  subjected  to  the 
action  of  breaking  waves.  On  the  other  hand  I  have  observed 
cases  in  which  it  seemed  to  me  the  phenomenon  was  caused  by  a 
single  backwash  current.  The  thin  sheet  of  water  returning 
down  the  beach  slope  appeared  to  be  split  into  diverging  minor 
currents  by  every  patch  of  more  compact  sand  or  particle  of 
coarser  material  which  impeded  its  progress,  and  the  crossing 
of  these  minor  currents  resulted  in  the  criss-cross  pattern  in  the 
sand.  Whatever  the  precise  mode  of  formation,  the  phenom- 
enon is  normally  the  product  of  backwash  from  waves  breaking 
on  the  beach  slope,  and  may  appropriately  be  called  backwash 
mark. 


518 


MINOR  SHORE  FORMS 


SAND  DOMES 

When  the  tide  is  advancing  up  the  slope  of  a  sandy  beach, 
and  the  swash  from  a  large  wave  first  sweeps  over  a  portion  of 
the  beach  previously  dry,  the  disappearance  of  the  water  may  be 
accompanied  by  the  appearance  of  miniature  domes  or  bhsters 
which  arise  at  various  points  over  the  area  newly  subjected  to 
the  action  of  the  swash.  These  domes  usually  vary  from  two  to 
eight  inches  in  diameter,  and  may  rise  an  inch  or  possibly  more 
above  the  level  surface  of  the  beach.  If  the  curious  observer  will 
gently  remove  one  side  with  a  knife  blade,  he  will  discover  that 
the  dome  is  hollow  as  shown  in  Figure  149,  the  vertical  height 
of  the  air  chamber  corresponding  to  the  height  of  the  dome 
surface  above  beach  level. 

The  formation  of  these  sand  domes  may  be  explained  as  fol- 
lows:  Before  the  swash  reaches  the  area  in  question,  the  beach 


Fig.  149.  —  Sand  dome. 


Arrov.s  show  movement  of  air  as  water  sinks 
down  from  surface. 


sands  are  dry,  and  air  fills  the  pore  spaces  between  the  sand 
grains.  The  first  advance  and  retreat  of  the  swash  saturates 
the  surface  layer  of  the  sand,  water  replacing  air  in  the  pore 
spaces  to  a  depth  of  one-fourth  or  one-half  inch.  Penetration 
of  the  water  to  greater  and  greater  depths  can  be  accomplished 
only  through  expulsion  of  the  air  previously  occupying  the  pore 
spaces.  Part  of  the  air  escapes  directly  through  the  surface 
film  of  wet  sand,  and  may  be  seen  bubbling  from  countless 
tiny  holes  before  the  swash  has  returned  down  the  beach.  In 
other  places  the  surface  film  of  wet  sand  is  quite  air-tight,  and  is 
locally  raised  as  a  perfect  miniature  dome  by  air  forced  upward 
through  the  action  of  water  descending  in  adjacent  areas.  Where 
the  waves  wash  a  layer  of  wet  sand  over  an  air-filled  hole  bored 


SAND  DOMES 


519 


XI 

H 


520  MINOR  SHORE  FORMS 

by  some  small  mollusc  the  formation  of  the  dome  may  be  facili- 
tated. It  is  hardly  necessary  to  remark  that  the  sand  domes, 
which  have  not  to  my  knowledge  been  previously  described,  are 
very  ephemeral  features. 

SHORE   DUNES 

The  sand  dunes,  formed  from  beach  sands  along  the  shore, 
have  received  much  attention  in  descriptions  of  shore  forms. 
They  are  extensively  developed  along  the  coast  of  the  Landes 
in  southwestern  France,  where  they  attain  heights  of  from  80 
to  90  meters  in  places,  cover  a  belt  from  2  to  6  miles  in  breadth, 
and  have  overwhelmed  houses  and  churches  causing  whole  towns 
to  be  abandoned  by  the  inhabitants^-^;  along  the  coast  of  the 
Netherlands  (Plates  LXX  and  LXXI)  and  Denmark,  where  they 
are  not  so  high  as  in  France,  but  nevertheless  serve  as  an  im- 
portant barrier  between  the  sea  and  the  lowlands  reclaimed  from 
tidal  waters,  attaining  a  height  of  30  meters  on  the  Danish  coast; 
and  on  the  south  and  east  coasts  of  the  Baltic,  where  they  cover 
broad  belts  on  the  Darss  foreland  and  near  Swinemiinde,  and 
rise  to  an  altitude  of  60  meters  on  the  narrow  bay  bars  of  the 
Frische  Haff  and  Kurische  Haff^-^.  On  the  Atlantic  coast  of 
the  United  States  shore  dunes  have  an  extensive  development 
near  Provincetown,  Massachusetts,  and  on  Cape  Canaveral, 
Florida;  while  smaller  areas  on  Sand}^  Hook  and  other  parts 
of  the  New  Jersey  coast ^^^,  near  Cape  Henry,  Virginia^^^,  and  on 
the  offshore  bars  of  the  Carolina  coast^"  are  noted  for  their 
dunes.  Inasmuch  as  these  dunes  are  the  product  of  wind 
action,  and  are  only  indirectly  related  to  shore  processes,  it  is 
not  desirable  to  consider  them  at  length  in  the  present  connec- 
tion. The  only  dunes  which  have  special  interest  for  the  student 
of  shore  processes  are  those  occurring  in  the  form  of  parallel 
ridges  on  a  beach  plain.  These  "  dune  ridges  "  have  already 
been  fully  discussed  in  Chapter  IX. 

The  student  desiring  to  pursue  further  the  study  of  dunes 
should  consult  the  early  work  of  Bremontier^^^  bearing  the  title 
"Memoire  sur  les  Dunes." 

Solger's  "Diinenbuch^^^,"  includes  a  treatment  of  shore  dunes, 
and  Sokolow,  in  his  important  wopk  entitled  "  Die  Diinen: 
Bildung,    Entwickelung,    und   innerer    Bau^^'^,"    discusses   sand 


SHORE   DUNES 


521 


K 


522 


MINOR  SHORE  FORMS 


X 

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X 

t        V    -* 

<; 

iJ 

Ph 

f' 

0 


X2 

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O 

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SHORE   DUNES  523 

dunes  of  all  types  and  gives  copious  references  to  the  literature 
of  the  subject.  He  reaches  the  conclusion  that  over  90  per  cent 
of  the  shore  dunes  of  Europe  occur  on  coasts  which  are  sub- 
siding, or  which  at  least  are  being  undermined  by  wave  attack; 
and  interprets  this  to  mean  that  the  undermining  action  con- 
stantly uncovers  fresh  supplies  of  sand  and  hinders  the  growth 
of  vegetation  which  might  protect  the  sand  from  wind  action, 
whereas  on  a  rising  coast  sand  deposits  may  be  raised  above 
the  reach  of  the  waves  and  be  replaced  by  clay  or  other  sand- 
free  sediments^^^ 

If  Sokolow's  conclusion  and  interpretation  were  valid,  the 
presence  or  absence  of  shore  dunes  would  become  a  matter  of 
much  importance  in  determining  past  changes  of  level.  Un- 
fortunately, the  criteria  accepted  by  this  author  as  satisfactory 
proofs  of  land  sinking  would  probably  result  in  the  classifica- 
tion of  90  per  cent  of  all  the  coasts  of  the  world  as  sinking  coasts; 
whereupon  the  occurrence  of  90  per  cent  of  the  dunes  upon 
such  coasts  would  lose  significance.  Neither  can  we  agree  that 
retrograding  coasts  necessarily  favor,  and  prograding  coasts 
hinder,  dune  formation.  The  almost  complete  absence  of 
shore  dunes  on  some  of  the  European  and  American  coasts 
suffering  most  from  wave  attack,  and  the  magnificant  develop- 
ment of  dunes  on  such  prograding  shores  as  those  of  the  Darss, 
Swinemiinde,  and  Cape  Canaveral,  point  to  a  different  inter- 
pretation. The  development  of  shore  dunes  depends  upon  a 
number  of  variable  factors,  among  which  are  the  direction  of 
the  wind  (offshore  or  onshore),  the  rapidity  with  which  debris 
is  suppHed  to  the  shore,  the  size  of  the  debris  particles,  the 
nature  of  the  climate,  and  the  stage  of  development  attained 
by  the  shoreline.  It  may  be  doubted  whether  very  slow  changes 
of  level  constitute  a  factor  of  importance.  In  any  case,  it 
would  seem  that  a  retrograding  shorehne,  along  which  more 
material  is  being  taken  from  the  land  than  is  added  to  it,  would 
present  conditions  unfavorable  to  the  extensive  accumulations 
of  shore  dunes;  whereas,  it  is  certain  that  the  dunes  of  the 
Darss,  Swinemiinde,  and  Canaveral,  and  probable  that  those 
of  the  Landes,  owe  both  their  formation  and  their  preservation 
to  the  prograding  of  sandy  shores. 


524 


MINOR  SHORE  FORMS 


X 
X 

a 

< 


'2 


O 


REFERENCES  525 


RESUME 


In  the  present  chapter  we  have  turned  our  attention  to  those 
minor  forms  of  the  shore  zone  which  have  no  great  significance 
in  the  general  history  of  the  shore  cycle,  but  which  nevertheless 
appeal  to  the  interest  of  every  observer  who  studies  the  meeting 
place  of  land  and  water  with  an  inquisitive  mind.  It  has  been 
shown  that  the  triangular  cusps  of  sand  or  gravel  built  by  waves 
upon  the  beach  have  given  rise  to  much  discussion  and  to  several 
theories  of  origin.  These  theories  we  have  examined  and  criti- 
cized in  the  light  of  new  evidence  as  to  the  distribution  and  char- 
acters of  the  cusps.  The  low  and  l)all  of  sandy  shores  have  been 
briefly  treated,  and  the  puzzling  problem  of  their  origin  indicated 
by  citations  from  different  observers.  We  have  examined  some 
of  the  rather  abundant  literature  relating  to  the  interesting  phe- 
nomena of  ripple  marks,  and  have  noted  the  value  which  these 
forms  have  to  the  geologist  who  must  interpret  the  origin  and 
structure  of  sedimentary  rocks.  Rill  marks,  swash  marks,  and 
the  marks  produced  by  the  backwash  in  turn  received  brief 
attention;  while  the  curious  but  very  temporary  sand  domes 
have  been  described  and  their  origin  explained.  Finally  the 
interesting  sand  dunes  occurring  on  the  shore  have  been  men- 
tioned, and  suggestions  offered  as  to  where  the  student  may  find 
elaborate  discussions  of  these  forms,  which  do  not  properly  lie 
within  the  province  of  a  book  devoted  to  shore  processes  and 
shoreline  development. 

REFERENCES 

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6.  .Jefferson,  M.  S.  W.     Beach  Cusps.     Jour,  of  Geol.     VII,  242,  1899. 

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8.  Branner,  J.  C.     Editorial  Note.     Jour,  of  Geol.     IX,  535,  1901. 

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Geol.  XI,  123,  1903. 


526  MINOR  SHORE  FORMS 

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23.  Shaler,   N.  S.     Beaches  and  Tidal  Marshes  of  the  Atlantic  Coast. 

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Crosby,  W.  O.  A  Study  of  the  Geology  of  the  Charles  River  Estuary  and 
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Davis,  W.  M.  Geographical  Essays.  Edited  by  Douglas  W.  Johnson,  777 
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Davis,  W.  M.  Dana's  Confirmation  of  Darwin's  Theory  of  Coral  Reefs. 
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Davis,  W.  M.  Shaler  Memorial  Study  of  Coral  Reefs.  Amer.  Jour.  Sci., 
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Dawson,  J.  W,  Acadian  Geology.  Third  Edition.  694  pp.,  and  supp.  102 
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Dawson,  W.  Bell.  Note  on  Secondary  Undulations  Recorded  by  Self- 
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Dawson,  W.  Bell.  Illustrations  of  Remarkable  Secondary  Tidal  Undula- 
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Delesse,  M.     Lithologie  des  Mers  de  France,  479  +  136  pp.,  Paris,  1872. 

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Dinse,  p.  Die  Fjordbildungen.  Ein  Beitrag  zur  Morphographie  der  Kiisten. 
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Dodge,  R.  E.  Continental  Phenomena  Illustrated  by  Ripple  Marks.  Science, 
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Duane,  J.  C,  et  al.  Report  of  Board  of  Engineers  on  Deepening  Gedney's 
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Ekman,  F.  L.  On  the  General  Causes  of  the  Ocean  Currents.  Nova  Acta 
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Ekman,  V.  W.     Ein  Beitrag  zur  Erklarung  und  Berechming  des  Stromver- 

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Ekman,  V.  W.     Beitrage  zur  Theorie  d      Meeresstromungen.     Annalen  der 

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Fischer,  Theobald.  Zur  Entwickelungsgeschichte  der  Kusten.  Petermanns 
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Fischer,  Theobald.  Kiistenstudien  aus  Nordafrika.  Petermanns  Geo- 
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Fischer,  Theobald.  Kiistenstudien  und  Reiseeindriicke  aus  Algerien. 
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Fleming,  J.  A.  Waves  and  Ripples  in  Water,  Air,  and  Aether.  299  pp., 
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FoREL,  F.  A.  Les  Rides  de  Fond  dans  le  Golfe  de  Morgues.  Bulletin  de  la 
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INDEX  — AUTHORS 


This  part  of  the  Index  includes  the  names  of  all  authorities  mentioned 
throughout  the  book. 

Often,  the  name  of  the  authority  will  not  be  found  upon  the  page  noted, 
—  only  the  number  of  that  authority  in  the  table  of  References  at  the  end  of 
the  chapter. 

Minor  heads  under  names  of  authorities  refer  to  general  subjects  treated, 
and  not  to  titles  of  authors'  works.  These  minor  heads  are  the  same  a3 
major  heads  under  Index  —  Subjects.  Under  those  major  heads  will  be 
found  the  names  of  all  the  authorities  upon  the  subjects. 


Abbe,  C, 

Development  of  Shoreline  — 
(Emergence) :  390,  394  {35) 
(Submergence) :  336,  347  {43) 
Abbe,  C,  Jr., 

Current  Action:  140,  141, 156  {153) 
Development  of  Shoreline  — 
(Emergence) :  383,  393  {27) 
Abercromby,  R., 

Water  Waves:  25,  50  {93) 
Agassiz,  a., 

Current  Action:  141,  156  {154) 
Minor  Shore  Forms:   402,  526  {10) 
Terminology  and  Classification  of 

Shores:  189 
Work  of  Waves:  80,  85  {52) 
Agassiz,  L., 

Development  of  Shoreline  — • 
(Emergence) :  350,  354,  392  (4) 
Airy,  G.  B., 

Current  Action:  143,  144 
Water  Waves:   3,  5,  12,  25,  29,  32, 
46  {8,  17),  48  {48),  50  {94),  51 
{114),  62  {123,  125) 
Work  of  Waves:  80,  85  {51) 
Anderson,  J., 

Current  Action:  136,  155  {137) 
Andrews,  E., 

Current  Action:  101,  149  {20) 
Development  of  Shore  Profile :  228, 

269  {28) 
Minor  Shore  Forms:  486,  526  {33) 


Andrews,  E.  C. 

Terminology  and  Classification  of 
Shores:   179,  196  {64,  68) 
Anonymous, 

Current  Action:  122,  153  {82) 
Antoine,  C, 

Water  Waves:  29,  51  {111) 
Appach,  F.  H., 

Shore  Ridges:  426,  455  {35) 
Austen,  R.  A.  C, 

Current  Action:  143 

Development  of  Shore  Profile:  210, 
268  (cS') 
Ayrton,  H., 

Minor  Shore  Forms:   498,  528  {59) 

Bache,  a.  D., 

Current  Action:  121,  140,  153  {75), 

156  {151) 
Water  Waves:   33,  52  {129) 
Bailey,  L.  W., 

Current  Action:  107,  150  {32) 
Barnes,  H.  T., 

Current  Action:  132,  155  {120) 
Barrell,  J., 

Development  of  Shoreline  — 
(Neutral  and  Compound):    397, 
403  {2) 
Minor  Shore  Forms:  491,  527  {46) 
Terminology  and  Classification  of 
Shores:     161,    165,    168,    193 
{6,  9,  21) 


553 


554 


INDEX  —  AUTHORS 


Bazin,  H., 

Water  Waves:    5,  18,  46  {16),  49 
{63) 
Beaufort,  F., 

Development  of  Shoreline  — 
(Submergence) :  309,  346  {IS) 
Beaumont,  E.  De, 

Development  of  Shoreline  — 
(Emergence):  351,354,355,350, 
358,  359,  365,  393  (,9) 
Development  of  Shore  Profile :  260, 
271  {82) 
Beaukain,  G., 

Minor    Shore    Forms:     519,    531 

{123) 
Shore  Ridges:   442,  456  {58) 
Beazeley,  a., 

Current  Action:    111,  151  (4<5) 
Beche,  H.  T.  De  La, 

Development  of  Shoreline  — 

(Submergence):  272,  345  {1) 
Minor  Shore  Forms:   491,  527  (4-5) 
Belcher,  E., 

Current  Action:  143 
Berendt,  G., 

Minor  Shore  Forms:  519,  531  {12^) 
Berghaus,  H. 

Terminology  and  Classification  of 
Shores:  170,  194  {36) 
Bertin,  E., 

Water  Waves:  5,  47  {21,  22) 
Bishop,    A.    L.,    Gregory,    H.    E., 
Keller,  A.  G.  and, 
Terminology  and  Classification  of 
Shores:  168,  194  {2^ 
Bjerknes,  V.  and  Sandstrom,  J.  W., 

Current  Action:  134,  155  {129) 
Bois,  C.  Des, 

Water  Waves:   23,  28,  50  {SO),  51 
{106) 

BOUSSINESQ,    J., 

Water  Waves:  5,  47  {20) 
Branner,  J.  C, 

Current    Action:     109,    119,    138, 

151  {J!^7),  152  {73),  156  {H3) 
Development  of  Shoreline  — 
(Submergence):    308,   309,   346 


Branner,  J.  C.  {continued), 

Minor  Shore  Forms:  460,  461,  462, 
475,  478,  501,  525  (7,  8),  526 
{27),  529  (74) 
Braun,  G., 

Minor  Shore  Forms:  487,  527  {37) 
Shore  Ridges:  422,  430,  455  {22) 
Bremontier,  N.  T., 

Current  Action:   104,  150  {2^) 
Minor    Shore    Forms:     519,    532 

{128) 
Water  Waves:  4, 11,  46  (H),  48  {^6) 
Brigham,  a.  P., 

Terminology  and  Classification  of 
Shores:  181,  196  {7J!^) 
Brogger,  W.  C, 

Terminology  and  Classification  of 
Shores:  179,  196  {62) 
Brown,  A.  P., 

IMinor  Shore  Forms:   501,  502,  529 
{76) 
Brown,  R., 

Terminology  and  Classification  of 
Shores:  181,  197  {81) 
Browne,  A.  J.  Jukes-;    see  Jukes- 
Browne,  A.  J. 
Browne,  W.  R., 

Current  Action:  113,  115,  119,  151 
(45),  152  {55,  61,  69) 
Brl'ckner, 

Shore  Ridges:  411,  435,  437,  438 
Bryson,  J., 

Development  of  Shoreline  — ■ 
(Emergence):  350,  392  (/,  2) 

BUCHAN,    A., 

Current    Action:     124,    136,    142, 
153  {90),  155  {139),  157  {161) 
Buchanan,  G.  Y., 

Current  Action:  124,  153  {89) 
Buchanan,  J.  Y., 

Current  Action:  139,  156  {150) 

BUCHER,   W.    H., 

Minor  Shore  Forms:  508,  529  {86) 
Bunt, 

Current  Action:  130,  154  {113) 
Burrows,  M., 

Shore  Ridges:    424,  426,  455  {32, 
36) 


INDEX  —  AUTHORS 


555 


Caligny,  a.  De, 

Current  Action:  93,  149  {15) 
Water  Waves:  3,  5,  6,  11,  32,  36, 

47  {26),  48  {45),  52  {127,  136) 
Calver,  E.  K., 

Work  of  Waves:  77,  85  {36) 
Candolle,  C.  De, 

Minor  Shore  Forms:  494,  495,  496, 
497,  498,  500,  509,  527  {54) 
Carpenter,  W.  B., 

Current  Action:  128,  154  {106) 
Case,  G.  O.,  Owens,  J.  S.  and. 

Current  Action:   97,  149  {17) 

ClALDI,    A., 

Water  Waves:  4,  5,  10,  18,  47  {25), 

48  {38,  42),  49  {67) 
Work  of  Waves:  80,  85  {53) 

Clapp,  C.  H., 

Development  of  Shoreline  — 
(Neutral  and  Compound):    401, 
403  U) 
Cobb,  C, 

Minor  Shore  Forms:  519,  532  {127) 
Cold,  C, 

Current  Action:  123,  153  {83) 
Development  of  Shoreline  — ■ 

(Submergence) :  309,  346  {19) 
Terminology  and  Classification  of 
Shores:  190,  198  {105) 

COMSTOCK,    F.    N., 

Development  of  ShoreHne  — 
(Submergence) :  335,  347  {39) 
CoNTE,  J.  Le;   see  Le  Conte,  J. 
COODE,   J., 

Current    Action:     144,    157    {170, 

178),  158  {183) 
Development  of  Shore  Profile:  217, 

219,  268  {10,  13) 
Work  of  Waves:  77,  80,  85  {37,  48) 

CORNAGLIA,    P., 

Current  Action:   91,  103,  149  (6'), 

150  {22) 
Water  Waves:  10,  48  {43) 
Cornish,  V., 

Current  Action:    89,  91,  107,  119, 

120,   121,   139,   149    {1,   7,  9), 

150   {28),   152   {66),   153    {74, 

77),  156  {145) 


Cornish,  V.  {continued), 

Minor    Shore    Forms:     460,    477, 

481,  488,  500,  501,  502,  525  (.?), 

526    {24,   29),    527    {40),    528 

{62,  63,  64,  65),  529  {78) 

Shore  Ridges:    411,  443,  455  {17), 

457  {60) 
Water  Waves:   3,  6,  12,  15,  18,  21, 
22,  24,  25,  26,  27,  28,  29,  46  {3, 
7),  47  {31),  48  {50),  49  {59,  65, 
72,  73,   75,   77),   50    {81,   83, 
85,  87,  88,  89,  90),  51  (,96,  97, 
103,  104,  107,  109,  110,  112) 
Work  of  Waves:    80,  82,  85  {50), 
86  {62,  63) 
Cotton,  C.  A., 

Development  of  Shoreline  — 
(Neutral  and  Compound):    397, 
403  (5) 
Terminology  and  Classification  of 
Shores:  189,  191,  198  {104,106) 
Credner,  G.  R., 

Development  of  Shoreline  — 
(Neutral  and  Compound):    395, 
403  {1) 
Cronander,  a.  W., 

Current    Action:     139,    142,    156 
{147),  157  {165) 
Crosby,  W.  O., 

Current  Action:  113,  119,  151  {51), 
152  {67) 

CUBITT,     ,7., 

Shore  Ridges:  411,  455  {16) 
Gushing,  H.  P., 

Minor  Shore  Forms:  509,  530  {100) 
Gushing,  S.  W., 

Development  of  Shore  Profile:  230, 
231,  270  {42,  43) 

Dall,  W.  H., 

Current  Action:  137,  141, 156'(U/, 

157) 
Daly,  R.  A., 

Development  of  Shore  Profile:  230, 

269  (34) 
Terminology  and  Classification  of 

Shores:     179,    189,    196    {67), 

198  {102) 


556 


INDEX  —  AUTHORS 


Dam  ANT,  Lt., 

Minor  Shore  Forms:  495 
Dana,  J.  D., 

Current  Action:  108,  126,  130,  145, 
151  (39),  154  {100,  110),  158 

Development  of  Shoreline  — 

(Submergence) :   272,  345  (3) 
Terminology  and   Classification  of 
Shores:    167,    174.    181,    189, 
193  (11),  195  (54),  196  {73) 
Darcy, 

Water  Waves:  5,  46  {16) 
Darwin,  G.  H., 

Minor  Shore  Forms:  495,  497,  498, 

508;   528  {58),  529  {S8) 
Terminology     and     Classification : 

174,  189 
Water  Waves:  43,  64  {163) 
Darwin,  L., 

Water  Waves:   28,  51  {107) 
Da  Vinci,  L.;  see  Vinci,  L.  da 
Davis,  C.  A., 

Development  of  ShoreUne  — 
(Emergence):     351,    354,     385, 
393  {31) 
Davis,  C.  H., 

Current  Action:   105,  150  {35) 
Davis,  W.  M., 

Development  of  ShoreHne  — 
(Submergence):    278,   281,   295, 
337,  339,  345  {3,  4,  9),  347  {U, 
46) 
Development  of  Shore  Profile :  203, 
223,  235,  245,  246,  247,  248, 
249,  253,  254,  256,  257,  258, 
260,  268  {1),  270  {59),  271  {66, 
67,  68,  69,  71,  73,  76,  77,  78, 
79,  80,  83,  85) 
Shore  Ridges:    405,  407,  408,  411, 

454  {99),  455  {13) 
Terminology  and  Classification  of 
Shores:  164,  167,  168,  169, 
172,  189,  193  {H,  19,  31,  33), 
194  {35,  36),  195  {51,  53), 
198  {101) 
Work  of  Waves:  73 


Davis,  W.  M.  and  Wood,  J.  W., 

Terminology  and  Classification  of 
Shores:  168,  194  {34) 
Dawson,  J.  W., 

Current  Action:  113,  114,  151  {53), 
152  {59,  60) 
Dawson,  W.  B., 

Current    Action:     130,    133,    154   ' 

{115),  155  {134) 
Water  Waves:   43,  54  {163) 
Delesse,  M., 

Work  of  Waves:   80,  85  {49) 
Des  Bois,  C;   see  Bois,  C.  des. 
Desor,  E., 

Minor  Shore  Forms:  486,  488,  526 
{31) 
Dinse,  p.. 

Terminology  and  Classification  of 
Shores:     181,    184,    196    {77). 
197  {98) 
Dodge,  R.  E., 

Minor  Shore  Forms:  489,  513,  627 
{43),  531  {117) 
Douglas,  J.  N., 
Current  Action:  143 
Work  of  Waves:   79,  85  {43) 
Drew,  F., 

Shore    Ridges:      404,     422,     424, 
426,  454  {3).  455  {35,  26,  37, 
33) 
DuANE,  J.  C,  el  al.. 

Development  of  Shoreline  — 
(Submergence):   300,  346  {11) 

DUTTON,    C.    E., 

Terminology  and  Classification  of 
Shores:  167,  193  {17) 

Ekman,  F.  L., 

Current  Action:  128,  130,  131, 
133,  134,  138,  139,  142,  154 
{105,  114,  116,117),  155  {131, 
137),  156  {144,  146),  157  {160, 
161,  163) 
Ekman,  V.  W., 

Current  Action:  89,  139,  149  {3), 
156  {148,  149) 

Water  Waves:   44,  54  {166) 

Work  of  Waves:  56,83  (1) 


INDEX  —  AUTHORS 


557 


Emy,  a.  R., 

Water  Waves:    4,  10,  11,  46  (13), 
48  (41,  47) 
Epry,  C, 

Minor  Shore  Forms:   502,  529  (77) 

ESMARK,    J., 

Terminology  and  Classification  of 
Shores:  179,  195  {59) 

EWART,    F.    C, 

Development  of  Shoreline  — 
(Submergence):   313 

Fairchild,  H.  L. 

Minor  Shore  Forms:  501,  517,  529 

(75),  531  (ISO) 
Terminology  and  Classification  of 
Shores:  181,  197  (80) 
Fenneman,  N.  M., 

Development  of  Shore  Profile:  211, 
220,  224,  235,  268  (3,  14),  269 
(/),  270  (61) 
Water  Waves:   13,  48  (52) 
Fischer,  T., 

Current  Action:   135,  155  (131) 
Development  of  Shore  Profile:  216, 
230,    268    (5),    269    (29,    30, 
31) 
Terminology  and  Classification  of 
Shores:     169,    176,    190,    194 
(30),  195  (57),  198  (105) 
Fleming,  J.  A., 

Water  Waves:   3,  6,  8,  12,  29,  30, 
46  {2,  4),  47  (2S,  35),  48  (4S), 
51  (115),  52  (119) 
Work  of  Waves:  56,  83  (1) 
Fleming,  S., 

Current  Action:  97,  149  (18) 
Development  of  Shoreline  — 
(Submergence):     292,    322,    345 
(8),  346  (26) 

FOERSTE,    A.    F., 

Minor  Shore  Forms:   509,  530  (97) 
FOREL,   F.  A., 

Minor  Shore  Forms:  494,  495,  496, 
497,   498,   512,   527   (55),   528 
(56,  57) 
Forbes,  E., 
Work  of  Waves:  82,  86  (59) 


FoL,  H., 

Work  of  Waves:  77,  85  (38) 

Gaillard,  D.  D., 

Current  Action:   93,  106,  126,  149 

(12),  150  (27),  153  (96) 
Water  Waves:  6,  13,  15,  20,  22,  23, 
24,  25,  26,  27,  30,  31,  32,  38, 
47  (33),  48  (53,  55),  49  (58,  71, 
74),   50    (79,   84,   92,   95),    51 
(98,  99),   52   (117,  121,  122), 
53  (153) 
Work  of  Waves:    56,  57,   62,   63, 
68,  83  (2,  3,  4),  84  (.9,  10,  11, 
12,  18,  21) 
Gallois,  L., 

Terminology  and  Classification  of 
Shores:  182,  197  (86) 
Gannett, 

Terminology  and  Classification  of 
Shores:  179 
Ganong,  W.  F.,  ^ 

Development  of  Shoreline  — 
(Emergence):      351,     354,     387, 
392  (7),  394  (33) 
Shore  Ridges:  446 
Gardiner,  J.  S., 

Current  Action:   109,  151  (46) 
Gaudry,  a.. 

Minor  Shore  Forms:  513,  531  (1 15) 
Geikie,  a.. 

Current    Action:     138,    144,    156 

(142),  157  (177) 
Development  of  Shore  Profile:  249, 

250,  271  (73,  74) 
Work  of  Waves:    68,  80,  84  (19, 
22),  85  (51) 
Gerstner,  F., 
Water  Waves:  4 

GiBBS,    J., 

Current  Action:  144 
Gilbert,  G.  K., 

Development  of  Shoreline  — 

(Emergence) :  352,  354,  355,  356, 

357,  358,  360,  365,   376,  393 

(11,  13,  14,  16) 

(Submergence):    287,   310,   322, 

336,  345  (5),  346  (:20),  347  (41) 


558 


INDEX  —  AUTHORS 


Gilbert,  G.  K.  {continued), 

Development  of  Shore  Profile:  259, 

260,  271  {8t,  8 It) 
Minor    Shore    Forms:    486,    488, 

494,  500,  501,  502,  508,  509, 

527  (54,  4<§),  529  {72,  80,  90), 

530  {98) 
Shore  Ridges:    405,  407,  408,  411, 

454  (7,  8,  12) 
Terminology  and  Classification  of 

Shores:     162,    163,    181,    193 

(7),  196  {69) 
Work  of  Waves:  69,  84  {23) 

GiLMORE,    J., 

Minor  Shore  Forms:   500,  528  {66) 

GOLDTHWAIT,    J.    W., 

Development  of  Shoreline  — 
(Emergence):   351,  392  (5) 
Shore  Ridges:    442,  446,  456  {56, 
59),  457  {61) 
Grabau,  a.  W., 

Curjent  Action:  124,  127,  128,  130, 
134,  136,  153  {87),  154  {110), 
155  {125,  130),  156  {im 
Minor    Shore    Forms:     513,    531 
{119) 
Green,  A.  H., 

Development  of  Shore  Profile :  235, 

270  {57) 
Terminology  and  Classification  of 
Shores:  176,  195  {55) 
Gregory,  H.  E.,  Keller,  A.   G.  and 
Bishop,  A.  L., 
Terminology  and  Classification  of 
Shores:  168,  194  {23) 
Gregory,  J.  W., 

Terminology  and  Classification  of 
Shores:     167,    182,    193    {12), 
197  (.9.3) 
Grossman,  K.  and  Lomas,  J., 
Terminology  and  Classification  of 
Shores:  181,  196  (75) 
Gulliver,  F.  P., 

Current    Action:     140,    141,    156 

{152,  151,) 
Development  of  Shoreline  — ■ 
(Emergence) :  376,  381,  382,  383, 
393  {22,  25,  28) 


Gulliver,  F.  P.  {continued), 

(Submergence):  291,  303,  308, 
311,  315,  .322,  .324,  328,  329, 
332,  3.33,  334,  339,  345  {6), 
346  {12,  16,  21,  23,  27,  28,  29, 
30,  31),  347  {32,  S3,  34,  45) 

Development  of  Shore  Profile:  225, 
226,  235,  269  {23,  25),  270  {60) 

Shore  Ridges:  404,  424,  426,  454 
{4),  455  {28,  30,  34) 

Terminology  and  Classification  of 
Shores:  159,  161,  164,  165, 
172,173,  192  {1,3),1^Z{5,8), 
195  {52) 

GURLT,    F.    A., 

Terminology  and  Classification  of 
Shores:  182,  197  {89) 

GtJTTNER,    P., 

Terminology  and  Classification  of 
Shores:  171,  181,  195  {48), 
197  {83) 

Haage,  R., 

Development  of  Shore  Profile :  234, 

270  {55) 
Termmology  and  Classification  of 

Shores:  169,  194  {32) 

Ha.\ST,    J.    VON, 

Terminology  and  Classification  of 
Shores:  179,  195  {60) 
Hagen,  G., 

Minor  Shore  Forms:   487,  527  {36) 
Water  Waves:  3,  18,  49  {61) 
Work  on  Waves:  57,  83  (5) 
Hahn,  F.  G., 

Development  of  Shore  Profile :  234, 

270  {54) 
Terminology  and  Classification  of 
Shores:    169,  171,  194  {31,  41)' 
Hallet,  H.  S., 

Current  Action:   108,  109,  151  {43) 
Hansen;  see  Helland-Hansen,  B. 
Harrington,  M.  W., 

Current  Action:  123,  126,  153  {85), 
154  {99) 
Harris,  R.  A., 

Current  Action:  108,  121,  122, 125. 
127,  128,  133,  135,  136,  140, 


INDEX  —  AUTHORS 


559 


142,  145,  151  (36),  153  (76,  78, 
92,  93),  154  {101,  102,  103, 
104),  155  (122,  123,  133,  135, 
UO),  156  (151,  158),  157  (166), 
158  (186) 
Water  Waves:  43,  53  (16  ■) 
Harrison*,  J.  T., 

Current  Action:  107,  150  (28) 
Work  of  Waves:   75,  85  (35) 
Haupt,  L.  M., 

Current  Action:   99,  149  (19) 
Water  Waves:  42,  63  (156) 
Hellaxd,  a., 

Terminology  and  Classification  of 
Shores:  179,  195  (61),  196  (61) 
Helland-Hansen,  B., 

Current  Action:  109,  135,  151  (U), 

155  (134) 
Development  of  Shore  Profile:  230 
Helland-Hansen,  B.  and  Hansen,  F. 
Current  Action:  89,  90,  149  (3,  4) 
Water  Waves:  44,  45,  54  (165,  167) 
Hentzschel,  O., 

Development  of  Shoreline  — ■ 

(Submergence):  306;  346  (14) 
Terminology  and  Classification  of 
Shores:     171,    190,    195    (47), 
198  (105) 
Henwood,  W.  J., 

Work  of  Waves:   71,  84  (5i) 
Hind,  H.  Y., 

Development  of  Shoreline  — 
(Submergence) :   292,  345  (7) 

HiRT,    O., 

Terminology  and  Classification  of 
Shores:  181,  196  (76) 
HiSE,  C.  R.  van;  see  Van  Hise,  C.  R. 
Hitchcock, 

Minor  Shore  Forms:   508 
Hjort,  J.,  Murray,  J.  and. 

Current  Action:  89,  109,  135,  136, 
141,  149  (3),  151  (44),  155 
(134,  126),  156  (156) 

lOBBS,   W.    H., 

Development  of  Shoreline  — 
(Submergence):   318,346(^5) 
Terminology   and    Classification 
of  Shores:    182,  197  (88) 


HoBBS,  W.  H.  (continued). 
Water  Waves:   38,  53  (139) 

Hole,  A.  D.,  Moore,  J.  and. 
Minor  Shore  Forms:   509,  530  (99) 

HOWLETT,    B.    S., 

Shore  Ridges:   411,  455  (15) 
Hubbard,  G    D., 

Terminology  and  Classification  of 
Shores:     179,    184,    196    (66), 
197  (99) 
Hull,  E., 

Terminology  and  Classification  of 
Shores:   181,  196  (75) 
Hunt,  A.  R., 

Current  Action:   124,  142,  144,  153 

(88),  157  (164,  169) 
Development  of  Shore  Profile:  216, 

268  (3,  7) 

Minor  Shore  Forms:   494,  495,  527 
(49,  50,  51,  52,  53) 

Water  Waves:  36,  52  (137) 

Work  of  Waves:   77,  82,  85  (39) 
Hunt.  E.  B., 

Current  Action:    141,  156  (154) 
Hyde,  J.  E., 

Minor  Shore  Forms:   505,  529  (85) 

Jagger,  T.   a.,  Jr., 

Mmor  Shore  Foi-ms:   509,  513,  530 
(92),  531  (118) 
Jefferson,  M.  S.  W., 

Minor  Shore  Forms:  460,  462,  463, 
467,  469,  470,  475,  477,  478, 
481,  525   (6,  9),  526  (12,  15, 
16,  17,  19,  22,  25,  26) 
Johis'son,  D.  W., 

Development  of  Shore  Profile :  224, 

269  (20),  247,  271  (70) 
Minor  Shore  Forms:   463,  510,  526 

(13),  530  (110) 
Terminology  and  Classification  of 
Shores:   182,  197  (94) 
Johnson,  D.  W.  and  Rekd,  W.  G., 
Development  of  Shoreline  — 

(Submergence):     295,    318,    346 
(10,  2J^) 
Development  of  Shore  Profile:  223, 
268  (18) 


560 


INDEX  —  AUTHORS 


Johnson,  D.  W.  and  Reed,  W.  G. 
{continued), 
Shore  Ridges:    412,  451,  455  (21), 
457  (62) 
Jukes-Browne,  A.  J., 

Development  of  Shore  Profile:  235, 

270  (68) 
Terminology  and  Classification  of 
Shores:   176,  195  (55) 

Kayser,  E., 

Development  of  Shore  Profile :  234, 
270  (52) 
Keilhack,  K., 

Shore  Ridges:    404,  411,  431,  433, 
435,   436,   437,   438,  440,   442, 
454  (6),  455  (20),  456  (42,  U, 
45,  48,  49,  50,  51,  53,  54,  57) 
Keller,  A.  G.  and  Bishop,  A.  L., 
Gregory,  H.  E., 
Terminology  and  Classification  of 
Shores:   168,  194  (23) 
Kelvin,  Lord, 

Water  Waves:  3,  8,  46  (i),  47  (35) 
Kemp,  J.  F., 

Minor  Shore  Forms:  466,  471,  488, 
526  (14,  20),  527  (42) 

KiNAHAN,    G.    H., 

Current  Action:  93, 105, 108, 144,149 

(13),  150(20),  151(37),  15S(181) 

Development  of  Shore  Profile:  216, 

268  (6) 
Work  of  Waves:  79,  85  (44) 
Kinahan,  H.  C, 

Current  Action:  108,  151  (38) 
Kindle,  E.  M., 

Minor  Shore  Forms:  500,  501,  504, 

505,  508,  517,    528    (67),   529 

(68,  69,  82,  83,  84),  530  (106, 

108),  531  (122) 

Kindle,  E.  M.  and  Taylor,  F.  B., 

Minor  Shore  Forms:  509,  530  (102) 

Kloden, 

Terminology  and  Classification  of 
Shores:  170 
Kornerup,  a., 

Terminology  and  Classification  of 
Shores:  182,  197  (91) 


KrIjger,  G., 

Current  Action:   126,  153  (97) 

Shore  Ridges:  437,  438.  456  (52) 
Krijmmel,  O., 

Current  Action:  94,  107,  108,  109, 
122,  129,  136,  150  (28,  30), 
151  (35,  40,  45),  153  (80), 
154  (107),  156  (140) 

Water  Waves:  3,  6,  15,  18,  20,  32, 
33,  35,  38,  39,  40,  41,  42,  43, 
46  (10),  47  (32),  49  (60,  69, 
70),  52  (128,  129,  134),  53 
(140,  14-5,  147,  148,  149,  150, 
151,  152,  153,  158,  159) 

Lagrange, 

Water  Waves:  32,  52  (126) 
Lane,  A.  C, 

Minor    Shore    Forms:     458,    476, 
525  (2) 
Lapparent,  a.  De, 

Development  of  Shore  Profile :  234, 

270  (50,  51) 
Terminology  and  Classification  of 
Shores:    167,    182,    193    (18), 
197  (85) 
Work  of  Waves:  82,  86  (61) 
Latrobe,  B.  H., 

Minor    Shore    Forms:     519,    532 
(126) 
Lawson,  a.  C, 

Development  of  Shore  Profile :  228, 

269  (27) 
Terminology  and  Classification  of 
Shores:  167,  193  (13) 
Le  Conte,  J., 

Current  Action:   138,  156  (143) 
Terminology  and  Classification  of 
Shores:     176,    182,    195    (58), 
197  (87) 
Lehmann,  F.  W.  p., 

Minor  Shore  Forms:   487,  527  (38) 
Lewin,  T., 

Shore  Ridges:   424,  455  (31) 
Lindenkohl,  a., 

Current  Action:  123,  125,  135, 
153  (84,  91),  155  (135) 


INDEX  —  AUTHORS 


561 


Livingston,  A.  A., 

Development  of  Shoreline  — 
(Submergence):  313 
Locke,  J., 

Minor  Shore  Fonns:  509,  530  {96) 
LoESCHE,    Pechuel-;     see    Pechuel- 

Loesche. 
LoMAS,  J.,  Grossman.  K.  and, 
Terminology  and  Classification  of 
Shores:  181,  196  {78) 
Lyell,  C, 

Minor  Shore  Forms:  497 
Work  of  Waves:  69,  71,  82,  84  {24, 
26) 
Lyman,  C.  S., 

Water  Waves:  8,48  {36) 

Marindin,  H.  L., 

Development  of  Shore  Profile:  223, 
268  {17) 
Marinelli,  O., 

Development  of  Shoreline  — 
(Submergence):  311 
Marsh,  G.  P., 

Water  Waves:  42,  53  {157) 
Marshall,  P., 

Terminology  and  Classification  of 
Shores:  181,  196  (70) 
Marten,  H.  J., 

Current  Action:    113,  151  {49) 
Martonne,  E.  De., 

Development  of  Shore  Profile:  234, 

270  {Jt9) 
Terminology  and  Classification  of 
Shores:     170,    171,    194    {W), 
195  {U) 
Marvine,  a.  R., 

Terminology  and  Classification  of 
Shores:  167,  193  {15) 
Matthews,  E.  R., 

Current  Action:  144,  157  {179) 
Work  of  Waves:  71,  84  {25,  27) 
Maury,  M.  F., 

Current  Action:   135,  155  {132) 
McGee,  W.  J., 

Development  of  Shoreline  — 
(Emergence):  351,354,387,388, 
392  {6),  394  {32,  34) 


Meinhold,  F., 

Terminology  and  Classification  of 
Shores:  169,  194  {34) 
Merrill,  B.  M., 

Development  of  Shoreline  — 
(Emergence) :  356,  370 
Merrill,  F.  J.  H., 

Development  of  Shoreline  — 

(Emergence) :   350,  354,  392  (5) 
Minor  Shore  Forms:  519,  531  {125) 
Meunier,  S., 

Work  of  Waves:  66,  84  {16) 
Mill,  H.  R., 

Current  Action:   142,  157  {159) 
Miller,  W.  J., 

Minor  Shore  Forms:  509,  530  {101) 
Mitchell,  H., 

Current    Action:     113,    119,    126, 
145,  146,  151  {52),  152  {66,  68, 
70,  71),  153  {95),  158  {185) 
Development  of  Shore  Profile:  237, 
270  {62) 

MoLLER, 

Water  Waves:   32,  52  {128) 
Moore,  J.  and  Hole,  A.  D., 

Minor  Shore  Forms:  509,  530  {99) 
Mottez,  a.. 

Water  Waves:   5,  28,  47  {24) 

MUDGE,    B.    F., 

Development  of  Shoreline  — 
(Emergence):   385,  393  {30) 
Murray,  J  , 

Current  Action:  93,  149  {11) 
Development  of  Shore  Profile:  216, 

268  {4) 
Terminology  and  Classification  of 

Shores:  189 
Work  of  Waves:  81,  85  {37,  55),  86 

{56) 
Murray,  J.  and  Hjort,  J.,  el  al., 
Current  Action:   89,  109,  135.  136, 

141,    149    {3),    151    {44),    155 

{134,  136),  156  {156) 
Terminology  and  Classification  of 

Shores:  189 

Nagel,  C.  H., 

Terminology     and     Classification: 
170,  194  (37) 


562 


INDEX  —  AUTHORS 


Nansen,  F., 

Development  of  Shore  Profile :  230, 
231,  270  (38,  41,  U,  45,  46) 
Work  of  Waves:   81,  85  [55) 
Nansen,   F.,   Helland-Hansen,   B. 
and 
Current  Action:   89,  90,  149  {3,  4) 
Water  Waves:  44,  45,  54  (165,  167) 
Nares,  Capt., 

Current  Action:    136,  155  (138) 
Nathorst,  A.  G., 

Minor    Shore    Forms:     513,     530 
(111,  112),  531  (114) 
Newton, 
Water  Waves :  4 

NORDENSKJOLD,    O., 

Terminology  and  Classification  of 
Shores:  182,  197  (95) 

NUSSBAUM,    F., 

Development  of  Shore  Profile :  230, 
270  (40) 

Oldham,  J., 

Current  Action:   111,  151  (48) 
Otto,  T., 

Current    Action:     134,    142,    155 

(126),  157  (167) 
Development  of  Shore  Profile :  223, 

269  (19) 
Minor  Shore  Forms:   487,  527  (39) 
Shore  Ridges:  404,  430,  435,  454 
(5),  456  (39,  40,  41,  46,  47) 
Owens,  J.  S., 

Minor  Shore  Forms:   501,  502,  529 
(75,  79) 
Owens,  J.  S.  and  Case,  G.  O., 
Current  Action:   97,  149  (17) 

Palmer,  H.  R., 

Current  Action:   96,  144,  149  (16), 

157  (174),  158  (182) 
Minor  Shore  Forms:   458,  525  (1) 
Paris,  A., 

Water  Waves:  27,  28,  51  (100, 109) 
Parsons,  H.  De  B., 

Current  Action:  107,  117,  122,  142, 
150  (31),  152  (62),  153  (79), 
157  (162) 


Passarge,  S., 

Terminology  and  Classification  of 
Shores:  167,  193  (20) 

PeCHUE  L-LOESCHE, 

Current  Action:   129,  154  (109) 
Penck,  a., 

Terminology  and  Classification  of 
Shores:     169,    181,    184,    194 
(29),  197  (84,  97) 
Pendleton,  A.  G., 

Development  of  Shore  Profile:  217, 
268  (12) 
Perkins,  F.  W., 

Current  Action:   141,  156  (155) 
Peschel,  O., 

Terminology  and  Classification  of 
Shores:  182,  197  (90) 
Petrocchi, 

Development  of  Shoreline  — 
(Submergence):  313 
Petterson,  O., 

Current    Action:     132,    134,    154 
(118),  155  (118,  128) 
Philippson,  a.. 

Current  Action:  94 
Philippson,  S., 

Development  of  Shoreline  — 
(Submergence) :  335,  347  (38) 

PlANIGIANI,    O., 

Development  of  Shoreline  — 
(Submergence):  312,  346  (22) 
Pierce,  R.  C, 

Minor  Shore  Forms:  500,  504,  529 
(70,  81) 
Playfair,  J., 

Terminology  and  Classification  of 

Shores:   176,  184,  195  (56) 
Work  of  Waves:  68,  84  {20) 
Powell,  J.  W., 

Terminology  and  Classification  of 
Shores:  167,  193  (16) 
Prestwich,  J., 

Current    Action:     144,    157    (171, 
172,  173,  175) 
Prosser,  C.  S., 

Minor    Shore    Forms:     509,    530 
{104) 


INDEX  —  AUTHORS 


563 


Ramsay,  A.  C, 

Development  of  Shore  Profile:  234, 
235,  270  {56) 
Range,  De, 

Development  of  Shoreline  — 
(Submergence):  335 
Rankine,  W.  J.  M., 

Development  of  Shore  Profile :  226, 

269  {2J^) 
Water  Waves:    5,  13,  47  {19),  48 
{51) 
Ratzel,  Fr., 
Terminology  and  Classification  of 
Shores:   159,  170,  184,  193  {I^), 
194  {39),  197  {100) 
Reade,  T.  I., 

Minor  Shore  Forms:  480 
Reade,  T.  M., 

Current  Action:  108,136,144,145, 
151  {41),  155  {137,  138),  158 
{ISj) 
Reclus,  E., 

Current  Action:  122,  153  {81) 
Redfield, 

Water  Waves:  24,  60  {82) 
Redman,  J.  B., 

Current  Action:    144,  157  {171) 
Shore  Ridges:    404,  411,  422,  424, 
454    {1,   3),   455    {H,   23,   2 4, 
29) 
Reed,  W.  G.,  Johnson,  D.  W.  and 
Development  of  Shoreline  — 
(Submergence):     295,    318,    346 
{10,  24) 
Development  of  Shore  Profile:  223, 

268  {18) 
Shore  Ridges:    412,  451,  455  {21), 
J        457  {62) 
Reid,  H.  F., 

Water  Waves:   39,  53  {142) 
Remmers,  O., 

Terminology  and  Classification  of 
Shores:  181,  197  {82) 
Rendel,  J.  M., 

Current  Action:  143 
Rennie,  G., 
Current  Action:  143 


Rettsch,  H., 

Development  of  Shore  Profile:  230, 

269  {35) 
Terminology  and  Classification  of 
Shores:  179,  196  {63) 
Reuschle, 

Terminology  and  Classification  of 
Shores:    171,  195  {4,5) 
Reynolds,  O., 

Minor  Shore  Forms:  498,  500,  501, 
528  {60,  61) 

RiCCHIERI,    G., 

Development  of  Shoreline  — 
(Submergence):  311 
Richardson, 

Current  Action:  108 
Richter,  E., 

Development  of  Shore  Profile :  230, 

269  {36) 
Richthofen,  F.  von. 

Development  of  Shorehne  — 

(Submergence) :  306,  346  {13) 
Development  of  Shore  Profile:  234, 

270  {47,  48) 
Terminology  and  Classification  of 

Shores:  169,  173,  194  {28) 
Riessen,  p., 

Terminology  and  Classification  of 
Shores:  171,  194  {42) 

RiTTER,    C, 

Terminology  and  Classification  of 
Shores:  170,  194  {35) 
Robertson,  W.  A.  S., 

Shore  Ridges:   426,  456  {37) 
Robinson, 

Current  Action:   93,  149  {12) 
Ross, 

Water  Waves:   28,  51  {105) 
Royal  Commission  on  Coast  Ero- 
sion, 
Work  of  Waves:  71,  84  {28) 
Russell,  I.  C, 

Development  of  Shoreline  — 

(Emergence):   352,  393  {12,  15) 
Minor  Shore  Forms:   486,  488,  526 
{30) 

RtJHL,    A., 

Current  Action:   129,  154  {108) 


564 


INDEX  —  AUTHORS 


Russell,  J.  S., 

Current  Action:  93,  104,  106,  107, 
143,  149  (10),  150  (24,  28) 

Water  Waves:  3,  4,  5,  8,  18,  32, 
33,  34,  35,  36,  37,  38,  39,  41, 
46  (5,  6,  9,  U,  15),  47  {35), 
49  {62,  64),  52  {124,  130,  131, 
132,  133),  53  {lb8,  I4I,  143, 
154) 

Saint-Venant,  B.  de. 

Water  Waves:  5,  47  {23) 
Salisbury,  R.  D., 

Current    Action:     119,    126,    130, 
152  {72),  154  {98,  110) 
Sandstrojm,  J.  W., 

Current  Action:   132,  155  {119) 

Water  Waves:  44,  54  {164) 
Sandstrom,  J.  W.,  Bjerknes,  V.  and. 

Current  Action:  134,  155  {129) 
Saporta,  G.  de. 

Minor  Shore  Forms:  513,  531  {113) 

SCHOTT,    A., 

Development  of  Shoreline  — 
(Emergence) :  350,  392  (3) 

ScHOTT,    G., 

Water  Waves:  27,  51  {101) 

SCHROTER,    W., 

Terminology  and  Classification  of 
Shores:  171,  195  {46) 

SCHWIND,    F  , 

Terminology  and  Classification  of 
Shores:     170,    171,    194    {38), 
195  {43) 
Scoresby,  W., 

Water  Waves:   24,  28,  50  {86,  88), 
51  {108) 
Scott,  W.  B., 

Development  of  Shore  Profile:  234, 
270  {53) 
ScROPE,  G.  p., 

Mmor  Shore  Forms:   508,  529  {87) 
Shaler,  N.  S., 

Current  Action:   93,  103,  149  {14) 
Development  of  Shoreline  — 

(Emergence):  351,354,355,380, 

393  {10,  19,  24) 
(Submergence) :  307, 335, 346  {15) 


Shaler,  N.  S.  {continued), 

Development  of  Shore  Profile :  261, 

271  {86) 
Minor  Shore  Forms:  459,460,469, 

476,  477,  481,  482,  525  {3,  4), 

526  {18,  23) 
Terminology  and  Classification  of 

Shores:  179,  196  {65) 
Work  of  Waves:   71,  84  {29,  30) 
Shannon,  W.  P., 

Minor  Shore  Forms:  509,  530  {109) 
Shield,  W., 

Current    Action:     130,    144,    154 

{112),  158  {180) 
Development  of  Shore  Profile:  217, 

268  {9) 
Work  of  Waves:   67,  80 

SlAU, 

Minor  Shore  Forms:  489,  491,  494, 

497,  504,  527  {44) 
Work  of  Waves:  80,  85  {54) 
Skertchley,  S.  B.  J., 

Current  Action:  113,  114,  151  {50), 
152  [57,  58) 
SOKOLOW,   N.  A.,  i 

Minor  Shore  Forms:   519,  523,  531 

{123,  124),  532  {130,  131) 
Shore  Ridges:  442,  456  {58) 
Solger,  F.,  et  al.. 

Minor  Shore  Forms:  519,  532  {129) 
Shore  Ridges:  442,  455  (LP), 456 (45) 

SOLLAS,    W.    J., 

Current  Action:  107,  108,  113,  114, 
117,  151  {34,  42),  152  {54,  56, 
64) 

SORBY,    H.    C, 

Minor  Shore  Forms:  494,  509,  527 

{47) 
Work  of  Waves:  82,  86  {60) 
Spratt, 

Current  Action:   144,  157  {176) 
Steffen,  H., 

Terminology  and  Classification  of 
Shores:  182,  197  {92) 
Stevenson,  R., 

Development  of  Shore  Profile :  237, 

271  {63) 
Work  of  Waves:  77,  79,  86  {40,  41) 


INDEX  —  AUTHORS 


565 


Stevenson,  T., 

Current  Action:  107,  126,  150  (33), 

153  (94) 
Water  Waves:  6,  15,  18,  23,  24,  47 

(27),  48  (.57),  49  (66),  50  (78, 

82) 
Work  of  Waves:  57,  G2,  63,  65,  74, 

79,  83  (6,  7),  84  (5,  ^?,  H,  15), 

85  (3^,  33,  3I^,  36,  J^2) 
Stokes,  G.  G., 

Current  Action:   90,  149  (5) 
Water  Waves:   5,  9,  10,  13,  45,  46 

(18),  47  (18),  48  (37,  39,  40, 

56),  54  (168) 
SUESS,   E., 

Terminology  and  Classification  of 

Shores:  169,  190,  194  (27) 

Tanner,  Z.  L., 

Water  Waves:  25..  50  (92) 
Tarr,  R.  S., 

Development  of  Shoreline  — 

(Submergence) :  334,  347  (35) 
Terminology  and  Classification  of 
Shores:  181,  196  (71) 
Tayler,  J.  W., 

Terminology  and  Classification  of 
Shores:  181,  197  (79) 
Taylor,  F.  B., 

Current  Action:    123,  153  (86) 
Water  Waves:  26 
Taylor,  F.  B.,  Kindle,  F.  M.  and, 
Minor  Shore  Forms:  509  530  (102) 
Thompson,  Sir  W.;  see  Kelvin.  Lord 
Thoflet,  J., 

Current  Action:    122,  153  (80) 
Water  Waves:    11,  18,  39,  43,  48 
(U),    49    (68),    53    (U6),    54 
(161) 
Work  of  Waves:   81,  86  (58) 
TOWNSEND,   C.   McD., 

Current  Action:    101,  150  (20) 

Udden,  J.  A., 

Minor  Shore  Forms:  509,  530  (103, 
107) 
Upham,  W., 

Terminology  and  Classification  of 
Shores:  181,  196  (73) 


Van  Hise,  C.  R., 

Minor  Shore  Forms:  508,  529  (8Q) 

Vaughan,  T.  W., 

Current  Action:    141,  156  (154) 
Terminology  and  Classification  of 
Shores:  189,  198  (103) 

ViXCI,    L.  DA, 

Water  Waves:  1,  4 
VOGT,  J.  H.  L., 

Development  of  Shore  Profile :  225, 

230,  269  (22,  37),  270  (30) 
Terminology  and  Classification  of 

Shores:  166,  193  (10) 

Weber,  E.  H.  and  W., 

Water  Waves:    4,^22,  39,  46  (12), 

49  (76),  53  (144) 
Work  of  Waves:   81,  86  (57) 
Weidemuller,  C.  R., 

Development  of  Shoreline  — 
(Emergence) :  355,  393  (20) 
Terminology  and  Classification  of 
Shores:     169,    171,    194    (34), 
195  (50) 
Werth,  E., 

Terminology  and  Classification  of 
Shores:    182,  197  (96) 
Weule,  K., 

Terminology  and  Classification  of 
Shores:    171,  195  (49) 
Wharton,  W.  J.  L., 

Development  of  Shore  Profile :  230, 
269  (32,  33) 
Wheeler,  W.  H., 

Current  Action:  103,  104,  107,  117, 
118,  119,  126,  130,  14?,  146, 
147,  150  (23,  24,  29),  152  (63, 
65,  66),  154  (97,  115),  157 
(168),  158  (187) 
Development  of  Shoreline  — 

(Submergence) :   335,  347  (40) 
Development  of  Shore  Profile:  217, 

220,  268  (11,  15,  16) 
Minor  Shor(>  Forms:   488,  527  (41) 
Shore  Ridges:    411,   442,  455  (18, 

55) 
Terminology  and  Classification  of 
Shores:  159,  192  {2) 


566 


INDEX  —  AUTHORS 


Wheeler,  W.  H.  {continued), 

Water  Waves:   4,  6,  35,  36,  37,  41, 
42,  47  (29,  30),  52  (134,  135), 
53  {150,  155) 
Workof  Waves:  79, 80, 85(4-5, 46, 47) 
Whewell,  W., 

Current  Action:   130,154(^3) 
White,  D., 

Development  of  Shoreline  — 
(Emergence):  351,  354 
White,  W.  H., 

Water  Waves:  6,  12,  13,  25,  27,  28, 
29,  30,  31,  47  {34),  48  {4-9,  54), 
50  {91),  51  {102,  105,  113),  52 
{116, 118, 120) 
Whittlesey,  C, 

Minor  Shore  Forms:  486,  526  {32) 
Williams,  H.  S., 

Development  of  Shore  Profile :  253, 
271  (75) 
Williamson,  W.  C, 

Minor  Shore  Forms:  513,  517,  531 
{116,  121) 


WiLUS,  B., 

Cm-rent  Action:  130,  154  {111) 
Wilson,  A.  W.  G., 

Current  Action:   101,  150  {21) 
Development  of  Shoreline  — 

(Submergence) :   335,  347  {37) 
Minor  Shore  Forms:  462,  463,  475, 
479,  480,  526  {11,21,  28) 
Wilson,  J., 

Current  Action:  143 
Wood,  J.  W.,  Davis,  W.  M.  and, 
Terminology  and  Classification  of 
Shores:  168,  194  {24) 
Woodman,  J.  E., 

Development  of  Shorelme  — 
(Submergence) :   334,  347  {36) 
Wooster,  L.  C, 

Minor    Shore    Forms:     509,    530 
{105) 
Wright,  W.  B., 

Development  of  Shore  Profile :  228, 
269  {,26) 


INDEX  — SUBJECTS 


This  part  of  the  Index  is  arranged  by  topics.  No  attempt  has  been 
made  to  inchide  general  or  casual  references. 

All  heads  have  been  based  upon  the  chapter  heads,  the  topical  heads,  and 
the  sub-topical  heads  of  the  book  itself.  At  the  end  of  those  heads  that  are 
the  same  as  the  minor  heads  of  the  Index  —  Authors,  special  reference  will 
be  found  to  the  authorities  upon  the  subjects. 


Abrasion,  marine,  theory  re,  234 

platform,  162,  225 
AeoUan  denudation,  166-169 

peneplane,  16(>-169 

plain,  164-169 

plane,  166-169 
Alluvial  outwash  plain,  263 

plain,  188 
Appach's  map,  of  ridges,  426 
Asymmetrical  ripples,  494-512 
see  also  Ripple  marks 

Backshore,  161 
teirace,  163 

see  also  Terraces 
Backwash,  517 

see  also  Marks 
Balls  and  lows,  486-489 
parallel  balls,  487-488 

see  also  Shore  forms,  minor 
Baltic  sea,  dunes  of  the,  519-524 
Barrier,  352 

see  also  Bars 
Barrier-bar,  352 
see  also  Bars 
Barrier-beach,  259 

see  also  Beaches 
Bars,  300-403 
barrier-bar,  352 
bay  bar,  300,  351 
bay-head  bar,  303 
bay-mouth  bar,  302 
compound  cuspate  bar,  322 


Bars,  cuspate  bar,  318 
cuspate  foreland  bar,  324 
cuspate  offshore  bar.  383 
flying  bar,  327 
looped  bar,  309 
marine  forces   in   development   of 

bars,  328 
marsh  bar,  325-327 
mid-bay  bar,  303 
offshore  bar,  259,  301,  350,  405 
submarine  bar,  349 
V-bar,  322 

see  also  Shoreline,  development 
of 
Bay  bar,  300,  351 
see  also  Bars 
delta,  328 

see  also  Deltas 
Bay-head  bar,  303 

see  also  Bars 
Bay-head  beach,  285 
see  also  Beaches 
Bay-mouth  bar,  302 

see  also  Bars 
Bay-side  beach,  285 
see  also  Beaches 
Beach  cusp,  224,  458-486 
see  also  Cusps,  beach 
drifting,  94-103 

see  also  Current  action 
plain,  297,  405 

profile  of  equilibrium,  217,  407 
see  also  Equilibrimu 


567 


568 


INDEX  —  SUBJECTS 


Beach  ridge,  297 

see  also  Shoreline,  development  of 
Beaches,  159-163,  215,  223,  259,  283- 
285 
barrier  beach,  259 
bay-head  beach,  285 
bay-side  beach,  285 
storm  beach,  223 

see   also    Shoreline   development 
of;  Terminology  and  classifica- 
tion of  shores 
Beach,  storm,  223 
Beaumont's,  de,  theory,  360 
Bench,  162,  203,  22-4,  258 

see  also  Shore   profile,   develop- 
ment   of;     Terminology    and 
classification  of  shores 
Berge,  ISIonadnock,  166 
Bottom  drag,  16 
Bomidary  waves,  2,  44-45 
see  also  Waves,  water 
Breaker,  16-20 
depth,  18-20 
see  also    Oscillation,    waves    of; 
Waves,  water 
Bruckner's  theory  of  35-year  cycle, 
411 

Canaveral,  Cape,  405,  519-524 
Capillar^'  waves,  7 

see     also      Oscillation,      waves; 
Waves,  water 
Carohnas,  dunes  of  the,  519-524 
Chesil  bank,  the,  217 

see  also  Shore   jjrofile,  develop- 
ment of 
Cinque  ports,  the,  424 
Classification  of  shores,  169-192 
genetic  methods,  170-171 
numerical  methods,  170-173 
compound  shores,  190-192 
emergence  shores,  186-187 
neutral  shores,  187-190 

{see  also  Neutral  shores) 
submergence  shores,  173-186 
(see  also  Submergence,  shores 
of;  Terminology  and  classi- 
fication of  shores) 


Chffs,  160-161,  203,  224,  259,  349 

retrograding  cliff,  295 
Coast,  160 
Coast-line,  159-160 
Coastal  plane,  166 
Combined  shore  profile,  265-266 

see  also   Shore   profile,  develop- 
ment of 
Combined  waves,  25-27,  36-38 

see  also   Oscillation,    waves    of; 
Waves,  water 
Combing  waves,  16 

see  also    Oscillation,    waves   of; 
Waves,  water 
Complex  cuspate  foreland,  325 
see  also  Forelands 
spit,  290 

see  also  Spits 
tombolo,  431 

see  also  Tombolos 
Compound  cuspate  bar,  322 

see  also  Bars 
Compound  recurved  spit,  290,  416- 
419 
Dune  ridge  spit,  418 
low  and  narrow  ridge  spit,  416 
parallel  ridge  spit,  416 
single  beach  spit,  419 
see  also  Spits 
Compound  shores,  190-192,  265-266, 
400-403 
see  also  Shore   profile,   develop- 
ment of;    Shoreline,  develop- 
ment  of,    Stages   in    develop- 
ment of  shores;    Terminology 
and  classification  of  shores 
Compound  spit,  405 

see  also  Spits 
Continental  shelf,  163  - 

terrace,  163 
Contraposed  shorelines,  401 

see  also  Shoreline,  development 
of 
Convection  current,  131 

see  also  Current  action 
Coral  reef,  188-189,  263 
see  also  Reefs 


INDEX  —  SUBJECTS 


569 


Current  action,  1,  87-158,  407 

beach  drifting,  94-103 

causes  of  currents,  87-88 

characteristics  of  currents,  1,  89 

complexities  of  current  action,  141- 
143 

conflicting     opinions     re     current 
action,  143-148 

convection  currents,  131 

debris  deposited  by  tidal  currents, 
113-115 

debris    moved    by    tidal    currents, 
115-121 

deflection  of  currents,  141 

eddy  currents,  139-141 

effects  of  longshore  currents,  222 

hydraulic  tidal  currents,  121-122 

hydraulic  wind  ciurents,  124—125 

longshore  currents,  407 

planetary  currents,  128-130 

pressure  currents,  130-131 

reaction  currents,  138-139 

river  currents,  136-138 

saUnity  currents,  131-136 
(see  also  Salinity  currents) 

seasonal  currents,  126-128 

Seiche  currents,  122-123 

temporary  currents,  125-126 

tidal  currents,  2,  106-122 
(see  also  Tidal  currents) 

types  of  currents,  87-90 

tvave  currents,  90-106 
(see  also  Wave  currents) 

wind  currents,  123-128 

{see  also  Wind  currents) 

References,  149-158 

see  also,  under  Index — Authors: 

Abbe,  C;    Agassiz,   A.;  Airy, 

G.  B.;  Andeison,  J.;  Andrews, 

E.;    Anonymous;    Austen,  R. 

A.  C;   Bache,  A.  D.;   Bailey, 

L,  W.;  Barnes,  H.  T.;  Beaze- 

ley,  A.;  Belcher,  E.;  Bjerknes, 

V.    and    Sandstrom,    J.    W.; 

Branner,    J.    C;    Breinontier, 

N.  T.;    Browne,  W,  R.;    Bu- 

chan.  A.;    Buchanan,  G.   Y.; 

Buchanan,  J.  Y.;   Bunt,  Calig- 


ny,  A.  de;  Carpenter,  W.  B.; 
Cold,  C;  Coode,  J.;  Comag- 
lia,  P.;  Cornish,  V.;  Cronan- 
der,  A.  W.;  Crosby,  W.  O. 
Dall,  W.  H.;  Dana,  J.  D. 
Davis,  C.  H.;  Dawson,  J.  W. 
Dawson,  W.  B.;  Douglas,  J 
N.;  Ekman,  F.  L.;  Ekman 
V.  W.;  Fischer,  T.;  Fleming 
S.;  Gaillard,  D.  D.;  Gardiner 
J.  S.;  Geikie,  A.;  Gibbs,  J. 
Grabau,  A.  W.;  Gulliver,  F 
P.;  Hallet,  H.  S.;  Harring- 
ton, M.  W.;  Harris,  R.  A. 
Harrison,  J.  T. ;  Haupt,  L.  M. 
Helland-Hansen,  B.;  Nansen 
F.;  Hunt,  A.  R.;  Hunt,  E.  B. 
Kinahan,  G.  H.;  Kinahan,  H 
C;  Kriiger,  G.;  Kriimmel,  O. 
Le  Conte,  J.;  Lindenkohl,  A. 
Marten,  H.  J.;  Matthews,  E 
R.;  Maury,  M.  F.;  Mill,  H 
R.;  Mitchell,  H.;  Murray,  J. 
Murray  J.  and  Hjort,  J.  et  al. 
Nares,  Capt.;  Oldham,  J. 
Otto,  T.;  Owens,  J.  S.  and 
Case,  G.  O.;  Palmer,  H.  R 
Parsons,  H.  de  B.;  Pechuel- 
Loesche;  Perkins,  F.  W.;  Pet- 
terson,  O.;  Pbilippson,  A. 
Prestwich,  J.;  Reade,  T.  M. 
Reclus,  E.;  Redman,  J.  B. 
Rendel,  J.  M.;  Rennie,  G. 
Richardson;  Robinson;  Riihl 
A.;  Russell,  J.  S.;  Salisbiu-y 
R.  D.;  Sandstrom,  J.  W 
Shaler,  N.  S.;  Shield,  W. 
Skertchley,  S.  B.  J.;  SoUas, 
W.  J.;  Spratt;  Stevenson,  T. 
Stokes,  G.  G.;  Taylor,  F.  B. 
Thoulet,  J.;  Townsend,  C 
McD.;  Vaughan,  T.  W. 
Wheeler,  W.H.;  Whewell,  W. 
Willis,  B.;  Wilson,  A.  W.  G. 
Wilson,  J. 

Current  ripples,  494-512 
see  also  Ripple  marks 

Currents,  see  Current  action 


570 


INDEX  — SUBJECTS 


Cuspate  bar,  318 
Cuspate  bar,  see  also  Bars 
compound,  322 
see  also  Bars 
delta,  409 

see  also  Deltas 
foreland,  322,  409 

see  also  Forelands 
foreland  bar,  324 

see  also  Bars 
offshore  bar,  383 
see  also  Bars 
Cusplet,  228,  479 

see  also  Cusps,  beach 
Cusps,  beach,  224,  458-486 
artificial  beach  cusp,  475 
characteristics  of  beach  cusp,  463 
cusplet,  479 

early  studies  re  beach  cusp,  458 
relation   of   beach   cusp   to   shore 

activity,  474 
theories  re  origin  of  beach  cusp,  476 
see  also  Shore  Forms,  Minor 
Cycle,  Bruckner's  35-year,  411 
Cycles  of  development,  228,  242-257 
emergence,  247 
fluvial,  242-245 
land-form,  247 
marine,  242-257 
shoreline,  247 
Cycloidal  waves,  13 

see  also  Oscillation,   waves    of; 
Waves,  water 

Darss,  the,  404,  428,  519-524 

see  also  Shore  forms,  minor 
Debris,  113-125 

Deposition  by  tidal  currents,  113-115 
movement  by  tidal  currents,  115- 
121 
see  also  Talus 
Deflection,  141,  307 

current  deflection,  141,  307 
stream  deflection,  307 
Deltas,  187-190,  263,  395 
bay  delta,  328 
cuspate  delta,  409 
tidal  delta,  374 


Deltas,  wave-delta,  306 

see  also  Shore  profile,  develop- 
ment of;    Shoreline,  develop- 
ment   of;     Terminology    and 
classification  of  shores 
Denmark,  dunes  of,  519-524 
Denudation,  166-169 
pluvio-fluvial,  166 
subaerial,  166-169 
Aeolian,  166-169 
fluvial,  166-169 
Deposition,  113-115,  162-163,  238 
backshore  terrace,  163 
beach,  159-163 
continental  shelf,  163 
continental  terrace,  163 
effect,  238 

shoreface  terrace,  163,  259 
tidal  currents,  113-115 
veneer,  163 
Deposits,  see  Deposition 
Depth,  of  break  wave,  18-20 

of  wave  action,  76-83 
Development    of   shore   profile;     see 
Shore  profile,  development  of 
stages  in;   see  Stages  in  develop- 
ment of  shores 
Development  of  shoreline;  see  Shore- 
line, development  of 
stages  in;  see  Stages  of  development 
of  shores 
Domes,  sand,  518-519 

see  also  Shore  forms,  minor 
Double  tombolo,  315 

see  also  Tombolos 
Drew's  map,  of  ridges,  422 
Drift,  shore,  259,  352 
Drifting,  beach,  94-103 
Drowned  valley,  272 
see  also  Valleys 
Dune  ridge,  404,  411,  418 

valley,  404 
Dunes,  shore,  519-524 

Baltic  Sea,  dunes  of  the,  519-524 
Canaveral,  dunes  of  Cape,  519-524 
Carolinas,  dunes  of  the,  519-524 
Darss,  dunes  of  the,  519-524 
Denmark,  dunes  of,  519-524 


INDEX  —  SUBJECTS 


571 


Dunes,  Landes,  dunes  of  the,  519-524 
Netherlands,  dunes  of  the,  519-524 
Provincetown,  dunes  of,  519-524 
sand  dunes,  519-524 
Sandy  Hook,  dunes  of,  519-524 
Swinemiinde,  dunes  of  the,  519-524 
wind  dunes,  519-524 

see  also  Shore  forms,  minor 

Dungeness,  the,  404,  419 
see  also  Ridges,  shore 

Dynamometer,  wave,  62-63 

Earthquake  and  explosion  waves,  2, 
38-41 
height,  40-41   (see  also  Height  of 

waves) 
motion,  38-40  (see  also  Motion  of 

waves) 
nature,  38-40 

origin,    38-40    (see  also  Origin   of 
waves) 
see  also  Waves,  water 
Eddy  currents,  139-141 

see  also  Current  action 
Elevation,  progressive,  386 
Embankment,  285 

Emergence,  shores  of,  186-187,  258- 
262,  348-391,  408 
see  also  Shore  profile,  develo{> 
ment  of;  ShoreUne,  develop- 
ment of;  Stages  in  develop- 
ment of  shoreUne;  Termi- 
nology and  classification  of 
shores 
Energy,  wave,  56-57 

conditions  affecting  energy,  72-74 
kinetic  energy,  56 
measurement  of  energy,  63-65 
potential  energy,  56 
see  also  Waves,  work  of 
Equilibrium,   beach   profile   of,    217, 
407 
profile  of,  225 
zone  of,  300 
Erosion  forms,  160-162 
abrasion  platform,  162 
bench,  162 
cliff,  160-161 


Erosion  forms,  see  also  Terminology 

and  classification  of  shores 
Explosion  waves,  earthquake  and,  2, 
38-41 
height,   40-41   (see  also  Height  of 

waves) 
motion,  38-40  (see  also  Motion  of 
,'         waves) 
nature,  38-40 

origin,    38-40    (see   also   Origin   of 
waves) 
see  also  Waves,  water 

Fault  shores,  189-190,  264,  397 

see  also  Neutral  shores 
Fetch,  22 
Fjord  shorelines,  176-186 

see  also  Submergence,  shores  of 
Fluvial  denudation,  166-169 
Fluvial  peneplane,  166 
plain, 164-169 
planation,  249 
Flying  bar,  327 

see  also  Bars 
Forelands,  322-325,  405 

complex  cuspate  foreland,  325 
cuspate  foreland,  322,  409 
simple  cuspate  foreland,  325 
truncated  cuspate  foreland,  325 
see  also  Shoreline,  development  of 
Foreshore,  161 

Form,  of  waves,  12-21,  33-34 
waves  of  oscillation,  12-21 
breaker,  16-20 
cycloidal,  13 
intersecting,  20-21 
surf,  16 
swell,  15 
trochoidal,  13 
waves  of  translation,  33-34 

see   also  Oscillation,    waves    of; 
translation  waves  of 
Formulae,  re  waves,  15,  20,  21,  23,  27, 

28,  31,  32,  56 
Frequency  of  waves,  30 
Fulcrum,  295 
FuUs,  404,  411 
neap  tide  full,  411 


572 


INDEX  —  SUBJECTS 


Fulls,  spring  tide  full,  411 

summer  full,  411 

winter  full,  411 

see  also  Ridges,  shore 
Furrow,  404 

Genetic  methods  of  classification  of 

shores,  170-173 
Gilbert's  theory,  re  bars,  360 
Glacial  peneplane,  166 

plain,  164-169 

plane,  166 
Groimd  swell,  15 

Hanging  valley,  343 
see  also  Valleys 
Headland,  winged,  303 
Height  of  waves,  21-27,  34 

oscillation,  21-27;    effect  of  fetch, 
22;    effect  of  wind  duration, 
22;    formute,  21,  23;    records, 
24 
translation,  34 

see   also    Oscillation,   waves    of; 
Translation,  waves  of;  Waves, 
water 
Hydraulic  currents,  121-125 
tidal,  121-122 
wind,  124-125 

see  also  Current  action 

Initial  stage;   see  Stages  in  develop- 
ment of  shores 
Inlets,  355 

migrating  inlet,  374 
tidal  inlet,  307,  367-368 

see  also  Shoreline,  development 
of 
Inshore,  161 
Intersecting  waves,  20-21 

see   also    Oscillation,   waves    of; 
Waves,  water 
Island,  formation  of,  272 

Kinetic  energy,  56 

see  also  Energy,   wave;    Waves, 
work  of 

Lagoon,  261,  350,  379 

Lames  de  Fond,  11 

Landes,  dunes  of  the,  519-524 


Length,  of  waves,  27-29,  35 
oscillation,  27-29 
records,  28-29 
waves,  translation,  35 

see  also    Oscillation,    waves    of; 
Translation,  waves  of;  Waves, 
water 
Level,  of  ridges,  439-453 
Lewin's  map,  of  ridges,  426 
Longshore  current,  222,  407 
Looped  bar,  309 

see  also  Bars 
Lows  and  balls,  486-489 
parallel  balls,  487-488 

see  also  Shore  forms,  minor 

Marks,  489-517 

backwash  mark,  517 
lill  mark,  512-513 
Ripple  mark,  489-512 
swash  mark,  513-517 

see  also  Shore  forms,  minor 
Marsh,  379 
Marsh  bar,  325 

see  also  Bars 
Mature  stage;  see  Stages  in  develop- 
ment of  shores 
INIeasurement,  of  wave  energy,  63-65 
Mid-bay  bar,  303 
see  also  Bars 
IMigratmg  inlet,  374 

see  also  Inlets 
Minor  shore  forms;  see  Shore  forms, 

minor 
IVlisdroy  spit,  431 
see  also  Spits 
Monadnock,  165-169 
Monadnock-Berge,  166 

see  also  Terminologj'  and  classi- 
fication of  shores 
Motion,  of  waves,  8-12,  34-35 
waves  of  oscillation,  8-12 
waves  of  translation,  34-35 

see  also  Oscillation,  waves  of; 
Translation,  waves  of;  Waves, 
water 

Nantasket  beach,  412 

see  also  Ridges,  shore 


INDEX  —  SUBJECTS 


573 


Neap  tide  full,  411 

see  also  Fulls 
Ness,  422 

Netherlands,  dunes  of  the,  519-524 
Neutral    shores,    187-190,    262-265, 
395-400 
alluvial  plain,  188 
coral  reef,  188-189 
delta,  187-190 
fault,  189-190 
outwash  plain,  188 
volcano,  188 

see  also  Shore  profile,  develop- 
ment of;  Shoreline,  develop- 
ment of;  Stages  in  develop- 
ment of  shores;  Terminology 
and  classification  of  shores 
Nip,  259,  349 

Numerical  methods  of  classification 
of  shores,  170-171 

Offset,  307 
Offshore,  161 

Offshore  bar,  259,  301,  350-405 
cuspate  offshore  bar,  383 

(see  also  Bars) 
development,  365 
not  evidence  of  subsidence,  380 
retrogression,  380 
see  also  Bars 
Old  stage;  see  Stages  in  development 

of  shores 
Orbits  of  waves,  11-12 
Origin^  of  waves,  1,  7-8,  33 
capillary  waves,  7 
earthquake   and  explosion  waves, 

38-40 
waves  of  oscillation,  7-8 
waves  of  translation,  8 

see  also    Oscillation,    waves    of; 
Translation,  waves  of;  Waves, 
water 
Oscillation,  waves  of,  1,  7-33 

depth   of   break,    18-20    (see   also 

Breaker) 
effect  of  mnd  upon  fetch,  22 
form  (see  also  Form  of  waves) 
formulae,  21-32 


Oscillation,  frequency,  30 

height,   21-27   (see  also  Height  of 

waves) 
length,  27-29   (see  also  Length  of 

waves) 
motion,  8-12   (see  also  Motion  of 

waves) 
orbits,  11-12 
origin,     7-8    (see    also    Origin    of 

waves) 
period,  30 
surf,  16 

swell  (ground-swell),  15 
velocity,  29-33  (see  also  Velocity  of 
waves);   breaker,  16-20;    com- 
bined   wave,    25-27;     combing 
wave,   16;    cycloidal  wave,   13; 
intersecting     wave,     20;      tro- 
choidal  wave,  13 
see  also  Waves,  water 
Oscillation  ripples,  494-512 

see  also  Ripple  marks 
Outwash  plain,  188 
Overlap,  307-308 

Peneplain,  159,  164-169 

aeolian,  164-169 

fluvial,  164-169 

glacial,  164-169 

marine,  164-169 
Peninsulas,  formation  of,  272 
Period,  of  waves,  30 
Plain,  159,  164-169 

aeolian,  164-169 

alluvial  outwash,  263 

beach,  297,  405 

fluvial,  164-169 

glacial,  164-169 

marine,  164-169 
see  also  Plane 
Planation,  199,  249-253 

fluvial,  249-253 

marine,  249-253 
Plane,  159,  164-169 

aeolian,  164-169 

coastal,  166 

fluvial,  164-169 

glacial,  164-169 


574 


INDEX  —  SUBJECTS 


Plane,  marine,  164-169 

see  also  Plaii. 
Planetary  currents,  128-130 

see  also  Current  action 
Platform,  abrasion,  162,  225 
Plu\ao-fluvial  denudation,  166 
Potential  energy,  56 

see  also  Energy,  wave;    Waves, 
work  of 
Pressure  currents,  130-131 

see  also  Current  action 
Profile  of  equilibrium,  225 
Prograding  shore,  223 
Progression     and     retrogression     of 

ridges,  409 
Progression  of  shore,  223 
Progressive  elevation  of  shore,  386 

subsidence  of  shore,  383 
Provincetown,  dunes  of,  519-524 

Reaction  currents,  138-139 
see  also  Current  action 
Recurved  spit,  290,  405 
see  also  Spits 
compound,  290,  416 
Reefs,  188,  259-308 
coral  reef,  188-189 
sand  reef,  259 
stone  reef,  308 

see  also  Shore   profile,  develop- 
ment of 
References,  current  action,  149-159 
shore  ridges,  454-457 
shore  fonns,  minor,  525-532 
shore  profile,  development  of,  268- 

271 
shoreline,  development  of: 
emergence,  392-394 
neutral  and  compoimd,  403 
submergence,  345-347 
terminologj"   and    classification   of 

shores,  192-198 
waves,  water,  46-54 
waves,  work  of,  83-86 

see  also  References,  mider  heads 
above,  for  authorities 
Refraction,  wave,  74-76 
see  also  Waves,  work  of 


Retrograding  cliff,  295 
see  also  Cliffs 

shore,  223 
Retrogression     and     progression     of 

ridges,  409 
Retrogression  of  offshore  bars,  380 

of  shores,  223 
Ria  shorelines,  173-176 

see  also  Submergence,  Shores  of 
Ridge,  single  beach,  419 
Ridges,  shore,  404-457 

Appach's  map  of  bridges,  426 

beach  plain,  405 

beach  ridge,  297 

beach  profile  of  equiUbrium,  407 

Briickner's  35-year  cycle,  411 

Cape  Canaveral,  405 

Cmque  Ports,  424 

complex  tombolo,  431 

compound  reciu-ved  spit,  416 

compound  spit,  405 

cuspate  delta,  409 

cuspate  foreland,  409 

Darss,  404,  428 

Drew's  map  of  ridges,  422 

Dune  ridge,  404,  411,  418 

dune  valley,  404 

Dungeness,  404,  419 

foreland,  405 

fulls,  404 

(see  also  Fulls) 

furrow,  404 

level  of  ridges,  439-453 

Lewin's  map  of  ridges,  426 

longshore  current,  407 

low  and  narrow  ridges,  416 

misdroy  spit,  431 

Nantasket  beach,  412 

Ness,  422 

offshore  bar,  405 

origm  of  ridges,  404-414 

parallel  ridges,  416 

progression    and    retrogression    Ci 
shores,  409 

rate  of  formation  of  ridges,  414;-439 

Rinnen,  422 

recurved  spit,  405 

Rockaway  beach,  416 


INDEX  —  SUBJECTS 


575 


Ridges,  shoreline  of  emergence,  408 
shoreline  of  submergence,  409 
single  beach  ridge,  419 
slash,  404 
swale,  404 

Swinemunde  spit,  431 
tombolo,  411 
Wallen,  422 
waves  of  translation, 
wave-terrace,  405 
References,  454-457 
see  also  under  Index  —  Authors  : 
Appach,  F.  H.;   Beaurain,  G.; 
Braun,    G.;     Bruckner;     Bur- 
rows, M.;    Cornish,  V.;    Cub- 
itt,  W.;  Davis,  W.M.;  Drew 
F.;    Ganong,  W.  F.;    GUbert 
G.    K.;     Goldthwait,    J.    W. 
Gulliver,  F.  P.;  Howlett,  B.  S. 
Johnson,  D.  W.  and  Reed,  W 
G.;     Keilhack,    K.;     Kriiger 
G.;     Lewin,    T.;     Otto,    T. 
Redman,    J.    B.;     Robertson 
W.  A.  S.;    Sokolow,   N.  A. 
Solger,    F.,    et    ai.;     Wheeler, 
W.  H. 
Rill  marks,  512-513 

see  also  Shore  forms,  minor 
Rills,  512-513 

see  also  Mark 
Rinnen,  the,  422 
Ripple  marks,  489-512 

asymmetrical  ripples,  494-512 
current  ripples,  494-512 
oscillation  ripples,  494-512 
symmetrical  ripples,  494-512 
theories  re  causes  of  ripple  mark, 

489 
tidal  ripples,  498-500 

see  also  Shore  forms,  minor 
Ripples,  489-512 

see  also  Marks 
River  currents,  136-138 

see  also  Current  action 
Rockaway  l)eacli,  416 

Salinity  currents,  131-136 

at  mouth  of  Baltic  Sea,  133-134 


Salinity  currents,  at   St.   of  Bab-el- 
Mandeb,  136 
at  St.  of  Gibraltar,  134-136 
see  also  Current  action 
Sand  domes,  518-519 

see  also  Shore  forms,  minor 
Sand  di^ne,  519-524 

see  also  Dunes,  shore 
Sand  reef,  259 

see  also  Reefs 
Sand  spit,  301 

see  also  Spits 
Sandy  Hook,  dunes  of,  519-524 
Seasonal  currents,  126-128 
see  also  Current  action 
Seiche  currents,  122-123 

see  also  Current  action 
Seiche  waves,  42-43 

see  also  Waves,  water 
Serpentine  spit,  291 

see  also  Spits 
Shelf,  continental,  163 
Shingle,  163 
Shore,  160 

Shore,  prograding,  223 
Shore,  retrograding,  223 
Shore  drift,  259,  352 
Shore  dunes,  519-524 

see  also  Dunes,  shore 
Shore  forms,  minor,  458-532 
backwash  marks,  517 
beach  cusps,  458-486 

(see  also  Casps,  beach) 
domes,  518-519 
dunes,  519-524 

(see  also,  Dunes,  shore) 
lows  and  balls,  486-489 

(see  also  Lows  and  balls) 
riU  marks,  512-513 
ripple  marks,  489-512 

(see  also  Ripple  marks) 
swash  marks,  513-517 
tidal  ripples,  498-500 
References,  525-532 
seealso,  under  Index— Authors: 
Agassiz,     A  ;      Andrews,     E.; 
Ayrton,  H.;  Barrell,  J.;  Beau- 
rain,  G.;    Beche,  H.  T.  de  la; 


576 


INDEX  —  SUBJECTS 


Berendt,  G.;    Branner,  J.  C 
Braun,  G.;  Bremontier,  N.  T 
Brown,  A.  P.;  Bucher,  W.  H 
Candolle,    C.    de;     Cobb,    C 
Cornish,  V.;    Gushing,  H.  P, 
Damant,  Lt.;   Darwin,  G.  H, 
Desor,    E.;     Dodge,    R.    E 
Epry,   C;    Fairchild,    H.   L, 
Foerste,  A.  F.;    Forel,  F.  A. 
Gaudry,  A.;    Gilbert,  G.  K. 
Gihnore,  J.;    Grabau,  A.  W 
Hagen,  G.;  Hitchcock;  Hunt 
A.  R.;    Hyde,  J.  E.;    Jagger 
T.  A.  Jr.;  Jefferson,  M.  S.  W. 
Johnson,  D.  W.;  Kemp,  J.  F. 
Kindle,  E.  M.;  Ivindle,  E.  M 
and  Taylor,  F.  B.;    Lane,  A 
C;  Latrobe,  B.  H.;  Lehmann 
F.W.  P.;  Locke,  J.;  Lyell,  C 
MerriU,  F.  J.  H.;    Miller,  W 
J.;  Moore,  J.  and  Hole,  A.  D. 
Nathorst,    A.    G.;     Otto,    T. 
Owens,  J.  S.;   Palmer,  H.  R. 
Pierce,  R.  C;    Prosser,  C.  S. 
Reade,  T.  I.;    Reynolds,  O. 
Russell,  I.  C;  Saporta,  G.  de 
Scrope,  G.  P.;    Shaler,  N.  S. 
Shannon,  W.  P.;   Siau;   Soko- 
low,  N.  A.;   Solger,  F.,  et  al; 
Sorby,  H.  C;    Udden,  J.  A.; 
Van    Hise,    C.    R.;     Wheeler, 
W.  H.;    Whittlesey,  C;    Wil- 
liamson,  W.   C;    Wilson,   A. 
W.  G.;  Wooster,  L.  C. 
Shore  profile,  development  of,   199- 
271 
compound  shores,  265-266 

stages  in  development,  265-266 
{see   also   Stages   in    develop- 
ment of  shores) 

see  also  Compound  shores 
emergence  shores,  258-262 

barrier  beach,  259 

lagoon,  261 

marine  bench,  258 

marine  chff,  259 

nip,  259 

offshore  bar,  259 


Shore  profile,  offshore  barrier,  259 
sand  reef,  259 
shore  drift,  259 
shoreface  terrace,  259 
stages  in  development,  258-262 

{see  also   Stages    in    develop- 
ment of  shores) 
see  also  Emergence,  shores  of 
neutral  shores,  262-265 
alluvial  outwash  plain,  263 
coral  reef,  263 
delta,  263 
fault,  264 
shoreface,  263 
stages  in  development,  262-265 

{see  also   Stages    in    develop- 
ment of  shores) 
see  also  Neutral  shores 
submergence  shores,  199-258 
abrasion  platform,  225 
beach,  215 
beach  cusp,  224 
beach     profile     of    equilibrium, 

217 
bench,  203,  224 
cliff,  203,  224 
cycles  of  development,  242-257 

{see  also   Cycles    of    develop- 
ment) 
effect  of  deposition,  238 

{see  also  Deposition) 
effects  of  longshore  currents,  222 

{see  also  Current  action) 
planation,  249 

{see  also  Planation) 
profile  of  equilibrium,  225 
prograding  shore,  223 
retrograding  shore,  223 
stages  in  development,  203-258  ~ 

{see  also   Stages    in    develop- 
ment of  shores) 
storm  beach,  223 
storm  terrace,  223 
talus,  203 
terrace,  224 

theory  of  marine  abrasion,  234 
theory  of  marine  cycle,  228 
wave  base,  225 


INDEX  —  SUBJECTS 


577 


Shore  profile,  see  also  Shoreline,  de- 
velopment   of;      Terminology 
and  classification  of  shores 
see  also  Submergence,  shores  of 
References,  268-271 
see  also  under  Index  — Authors 
Andrews,  E.;    Austen,  R.  A 
C;   Beaumont,  E.  de;   Coode 
J.;   Gushing,  S.  W.;    Daly,  R 
A.;  Davis,  W.M.;  Fenneman 
N.  M.;  Fischer,  T.;  Geikie,  A. 
Gilbert,  G.  K.;   Green,  A.  H. 
Gulliver,    F.    P.;    Haage,    R. 
Hahn,  F.  G.;  Helland-Hansen 
B.;  Hunt,  A.  R.;  Johnson,  D 
W.;  Johnson,  D.  W.  and  Reed 
W.  G.;    Jukes-Browne,  A.  J. 
Kayser,  E.;    Kinahan,  G.  H. 
Lapparent,    A.    de;     Lawson, 
A.  C.;  Marindin,  H.  L.;  Mar- 
tonne,    E.    de;    Mitchell,    H.; 
Murray,  J.;  Nansen,  F.;  Nuss- 
baum,  F.;    Otto,  T.;  Pendle- 
ton, A.  G.;    Ramsay,  A.  C. 
Rankine,  W.  J.  M.;  Reed,  W 
G.;   Reusch,  H.;   Richter,  E. 
Richthofen,  F.  von;  Scott,  W 
B.;  Shaler,  N.  S.;  Shield,  W. 
Stevenson,  R.;  Vogt,  J.  H.  L. 
Wharton,  W.  J.  L.;    Wheeler 
W.     H.;     Williams,     H.     S. 
Wright,  W.  B. 
Shore  ridges,  404-457 
see  Ridges,  shore 
Shoreface,  263 

terrace,  163,  259 
Shoreline,  159 

high  tide  shoreline,  161 
low  tide  shoreline,  161 
Shoreline,  development  of,  272-403 
compound  shorelines,  400-401 
(see  also,  beloiv,  Neutral  and  com- 
pound shorelines) 
contraposed  shorelines,  401-403 
(see  also,  below,  Neutral  and  com- 
pound shorelines) 
emergence  shorelines,  348-394 


Shoreline,  emergence,  bars,  350-390 

(see  also  Bars) 
barrier,  352 
barrier-bar,  352 
bay  bar,  351 
Beaumont's,  de,  theory  re  shores, 

360 
cliff,  349 

cuspate  offshore  bar,  383 
Gilbert's  theory  re  shores,  360 
inlets,  355 

(see  also  Inlets) 
key,  351 
lagoon,  350,  379 
marsh,  379 
migrating  inlet,  374 
nip,  349 

offshore  bar,  350-390 
progressive  elevation,  386 
progressive  subsidence,  383 
shore  drift,  352 

stages    in    development,    initial 
stage,  348-350 

young  stage,  350-389 

mature  stage,  389-390 

old  stage,  390-391 

(see  also  Stages  in  develop- 
ment of  shores) 
submarine  bar,  349 
tidal  delta,  374 
tidal  inlet,  367 

see  also  Emergence,  shores  of 
References,  392-394 
see  also,  under  Index — Authors 

Abbe,    C.    Jr.;     Agassiz,    L. 

Beaumont,  E.  de;   Bryson,  J. 

Davis,  C.  A.;   Davis,  W.  M. 

Ganong,  W.  F.;    GQbert,  G 

K.;    Goldthwait,  J.  W.;    Gul- 
liver, F.  P.;    McGee,  W.  J.; 

Merrill,  B.  M.;   Merrill,  F.  J. 

H.;  Mudge,  B.  F.;  Russell,  I. 

C.;  Schott,  A.;  Shaler,  N.  S.; 

WeidcmiiUer,  C.  R.;  White,  D. 
Neutral  shorelines,  395-400 

(see    also,    below,    Neutral    and 

compound  shorelines) 
delta,  395 


578 


INDEX  —  SUBJECTS 


Shoreline,  neutral,  fault,  397 

Neutral  and  compound  shorelines, 

395-403 
compound  shorelines,  400-401 
contraposed  shorelines,  401-403 
neutral  shorelines,  395-400 
stages  in  development,  395-403 

(see  also   Stages   in    develop- 
ment of  shores) 

see    also    Neutral    and    com- 
pound shores 
References,  403 
seealso,  under  Index — Authors: 

BarreU,    J.;     Clapp,    C.    H.; 

Cotton,  C.  A.;  Credner,  G.  R. 
submergence  shorelines,  272-347 
bars,  300-340  (see  also  Bars) 
bay  bar,  300 
bay  delta,  328 
bay-head  bar,  303 
bay-head  beach,  285 
bay-mouth  bar,  302 
bay-side  beach,  285 
beaches,  283-300 

(see  also  Beaches) 
beach  plain,  297 
beach  ridge,  297 
complex  cuspate  foreland,  325 
complex  spit,  290 
compound  cuspate  bar,  322 
compound  recurved  spit,  290 
cuspate  bar,  318 
cuspate  foreland,  322 
cuspate  foreland  bar,  324 
deltas,  306,  328 

(see  also  Deltas) 
double  tombolo,  315 
drowned  valley,  272 
embankment,  285 
flying  bar,  327 
forelands,  322-325 

(see  also  Forelands) 
fulcrum,  295 
hanging  valley,  343 
island,  272 

irregular  sea-bottom  and  shore- 
line, 272 
looped  bar,  309 


Shoreline,  submergence,  marsh   bar, 
325 
mid-bay  bar,  303 
offset,  307 
offshore  bar,  301 
overlap,  307 
peninsula,  272 
recurved  spit,  290 
retrograding  cliff,  295 
sand  spit,  301 
serpentine  spit,  291 
siinple  cuspate  foreland,  325 
spits,  287-302 

(see  also  Spits) 
stages    in    development,    initial 
stage,  272-275 

young  stage,  275-339 

mature  stage,  339-344 

old  stage,  344 

(see  also  Stages  in  develop- 
ment of  shores) 
stone  reef,  308 
stream  deflection,  307 
tombolos,  310-320 

(see  also  Tombolos) 
tidal  inlet,  307 

truncated  cuspate  foreland,  325 
Valleuse,  343 
valleys,  272,  343 

(see  also  Valleys) 
V-bar,  322 
wave-delta,  306 
winged  headland,  303 
Y-tombolo,  315 
zone  of  equilibrium,  311 

see  also  Submergence,  shores  of 
References,  345-347 
see  aZso,  under  Index — Authors: 

Abbe,  C;  Beaufort,  F.;  Beche, 

H.    T.    de    la;    Branner,     J. 

C;    Cold,  C;    Comstock,  F. 

N.;    Dana,  J.  D.;    Davis,  W. 

M.;     Duane,    J.    C,    et    al.; 

Ewart,    F.    C;     Fleming,    S.; 

Gilbert,  G.  K.;    GuUiver,  F. 

P.;   Hentzschel,  O.;   Hind,  H. 

Y.;    Hobbs,  W.  H.;    Johnson, 

D.    W.    and    Reed,    W.    G.; 


INDEX  —  SUBJECTS 


579 


Livingston,  A.  A.;  Marinelli, 
O.;  Petrocchi;  Philippson,  S.; 
Pianigiani,  O.;  Ranee,  de; 
Riechieri,  G.;  Richthofen,  F. 
von;  Shaler,  N.  S.;  Tarr,  R. 
S.;  Wheeler,  W.  H.;  Wilson, 
A.  W.  G.;  Woodman,  J.  E. 
Shorelines,  delta,  395 

fault,  397 
Shores,  classification  of,  see  Classifi- 
cation of  shores;  Terminology 
and  classification  of  shores 
compound,  190-192,  265-266,  400- 
403 
see  also   Shore  profile,   develop- 
ment of;    Shoreline,   develop- 
ment of;     Stages  in  develop- 
ment of  shores;    Terminology 
and  classification  of  shores 
emergence,  186-187,  258-262,  348- 
391 
see  also  Shore   profile,   develop- 
ment of;    Shoreline,   develop- 
ment of;    Stages  in  develop- 
ment of  shores;    Terminology 
and  classification  of  shores 
neutral,  187-190,  262-265,  395-400 
see  also  Shore  profile,   develop- 
ment of;    Shoreline,    develop- 
ment of;    Stages  in  develop- 
ment of  shores;    Terminology 
and  classification  of  shores 
submergence,     173-186,     199-258, 
272-344 
see  also  Shore   profile,  develop- 
ment of;    Shoreline,   develop- 
ment     of;      Stages     in      de- 
velopment   of     shores;     Ter- 
minology and  classification  of 
shores 
terminology  and  classification  of; 
see  Terminology  and  classifi- 
cation of  shores 
terminology  of;    see  Terminology 
of  shores;     Terminology  and 
classification  of  shores 
Simple  cuspate  foreland,  325 
see  also  Forelands 


Single  beach  ridge,  419 

see  also  Ridges,  shore 
Slash,  404 

Spits,  287-300,  404-439 
complex  spit,  290 
compound  spit,  405 
(impound  recurved  spit,  290,  416 
(see  •  also    Compound    recurved 
spit) 
misdroy  spit,  431 
recurved  spit,  290,  405 
sand  spit,  301 
serpentine  spit,  291 
Swinemiinde  spit,  431 

see  also  Shoreline,   development 
of;  Ridges,  shore 
Spring  tide  full,  411 

see  also  Fulls 
Stages  in  development  of  shores,  190, 
201-266,     272-344,     348-391, 
395-403 
initial    stage;     compound    profile, 
265-266 
compound  shoreline,  400-403 
emergence  profile,  258-259 
emergence  shoreline,  348-350 
neutral  profile,  262-265 
neutral  shoreline,  395-400 
submergence  profile,  201-203 
submergence  shoreline,  272-275 
young    stage;     compound    profile, 
265-266 
compound  shoreline,  400-403 
emergence  profile,  259-262 
emergence  shoreline,  350-389 
neutral  profile,  262-265 
neutral  shoreline,  395-400 
submergence  profile,  203-210 
submergence  shoreline,  275-339 
mature  stage;     compound  profile, 
265-266 
compound  shoreline,  400-403 
emergence  profile,  262 
emergence  shoreline,  389-390 
neutral  profile,  262-265 
neutral  shoreline,  395-400 
submergence  profile,  210-224 
submergence  shoreline,  339-344 


580 


INDEX  — SUBJECTS 


Stages  in  development  of  shores,  old 
stage;   compound  profile,  265- 
266 
compound  shoreline,  400-403 
emergence  profile,  262 
emergence  shoreline,  390-391 
neutral  profile,  262-265 
neutral  shoreline,  395-400 
submergence  profile,  224-258 
submergence  shoreline,  344 
see  also  Shore  profile,  develop- 
ment of;  Shoreline,  develop- 
ment of;  Terminology  and 
classification  of  shores 
Standing  waves,  42-43 
seiches,  42-43 

see  also  Seiches;  Waves,  water 
Storm  beach,  223 

see  also  Beaches 
Storm  terrace,  223 

see  also  Terraces 
Storm  waves,  damage  done  by,  65-72 

see  also  Waves,  work  of 
Strand,  159 
Strandline,  159 
Stream  deflection,  307 
Subaerial  denudation,  166 
aeolian,  166 
fluvial,  166 
Submarine  bar,  322,  349 

see  also  Bars 
Submergence,     shores    of,     173-186, 
199-258,  272-344 
Ria  shorelines,  173-176 
Fjord  shorelines,  176-186 

see  also  Shore   profile,  develop- 
ment of;    Shoreline,   develop- 
ment   of;    Stages  in  develop- 
ment   of     shoreline;     Termi- 
nology   and    classification    of 
shores 
Subsidence,  progressive,  383 
Summer  full,  411 
see  also  Fulls 
Surf,  16 
Swale,  404 
Swash,  513-517 


Swash  marks,  513-517 

see  also  Marks,  minor 
Swell,  15 

Swinemiinde,  dunes  of  the,  519-524 
spit,  431 
tombolo,  431 
Symmetrical  ripples,  494-512 
see  also  Ripple  marks 

Talus,  203 

see  also  Debris 
Temporary  currents,  125-126 

see  also  Current  action 
Terminology    and    classification    of 
shores,  159-198 
abrasion  platform,  162,  225 
backshore  terrace,  163 
beach, 159-163 
bench,  162 
classification  of  shores,  169-192 

(see  also  Classification  of  shores) 
cliff,  160-161 
coast,  160 

compound  shores,  190-192 
continental  shelf,  163 

terrace,  163 
coral  reef,  188-189 
delta,  187-190 
denudation,  166-169 

(see  also  Denudation) 
deposition,  113-115,  162-163,  238 
'     (see  also  Deposition) 
emergence  shores,  186-187 
erosion  forms,  160-162 

(see  also  Erosion  forms) 
fault  shores,  189-190 
fjord  shorelines,  176-186 
genetic  methods  of  classification  of 
shores,  170-173 

(see  also  Classification  of  shores) 
inshore,  161 
Monadnock,  165-169 
neutral  shores,  187-190 

(see  also  Neutral  shores) 
numerical  methods  of  classification 
of  shores,  170-171 

(see  also  Classification  of  shores) 
offshore,  161 


INDEX  —  SUBJECTS 


581 


Terminology     and     classification    of 
shores,    peneplain,    peneplane, 
plain,  plane,  159,  164-169 
(see   also  Peneplain,   Peneplane, 
Plain,  Plane) 

Ria  shorelines,  173-176 

shingle,  163 

shore,  160 

shoreface  terrace,  163,  259 

shoreline,  159 

stages  in  development.  190 

submergence  shores,  173-186 
(see  also  Submergence,  shores  of) 

terminology  of  shores,  159-169 
(see  also  Terminology  of  shores) 

veneer,  163 

volcano  shores,  188 

water  line,  159 

zones,  159-161 
(see  also  Zones) 
References,  192-198 
see  also,  under  Index — Authors 
Agassiz,  A.;    Andrews,  E.  C. 
Barrell,     J.;      Berghaus,     H. 
Brogger,  W.  C;    Brown,  R. 
Cold,     C;      Cotton,     C.     A. 
Daly,    R.    A.;     Dana,    J.    D. 
Darwin,  G.  H.;  Davis,  W.  M. 
Davis,  W.  M.  and  Wood,  J 
W.;  Dinse,  P.;  Dutton,  C.  E. 
Esmark,  J.;    Fairchild,  H.  L. 
Fischer,  T.;   Gallois,  L.;  Gan- 
nett;   Gilbert,  G.  K.;    Green, 
A.     H.;      Gregory,     H.     E., 
Keller,  A.  G.  and  Bishop,  A. 
L.;    Gregory,   J.   W.;    Gross- 
man, K.  and  Lomas,  J.;   Gul- 
Uver,    F.    P.;     Gurlt,    F.    A. 
Guttner,     P.;       Haage,     R. 
Haast,  J.  von;    Hahn,  F.  G. 
Helland,  A.;    Hentzschel,  O. 
Hirt,0.;  Hobbs,  W.  H.;  Hub- 
bard, G.  D.;    Hull,  E.;  John- 
son,   D.    W.;     Jukes-Browne, 
A.  J.;  Kloden;  Kornerup,  A.; 
Laparent,  A.  de;    Lawson,  A. 
C;    Le  Conte,  J.;    Marshall, 
P.;    Martonne,  E.  de;    Mar- 


vine,    A.    R.;     Meinhold,    F.; 
Murray,  J.;    Murray,   J.  and 
Hjort,  J.  ct  al.;  Nagel,  C.  H. 
Nordenskjold,    O.;     Passarge 
S.;    Penck,   A.;    Peschel,   O. 
Playfair,    J.;     Powell,   J.   W. 
Ratzel,    Fr.;     Remmers,    O. 
Reusch,  H.;    Rcuschle;    Rich- 
thofen,  F.  von;    Riessen,  P.; 
Ritter,      C;      Schroter,    W.; 
Schwind,   F.;    Shaler,   N.   S.; 
Steffen,  H.;  Suess,  E.;   Tarr, 
R.  S.;  Tayler,  J.  W.;  Upham, 
W.;  Vaughan,  T.  W.;  Vogt,  J. 
H.    L.;    WeidemuUer,   C.    R.; 
Werth,  E. ;  Weule,  K. ;  Wheeler, 
W.  H. 
Terminology  of  shores,  159-169 
denudation,  166-169 

(see  also  Denudation) 
deposition,  159-164 

(see  also  Deposition) 
erosion  forms,  160-162 

(see  also  Erosion  forms) 
peneplanes,  164-169 

(see  also  Peneplanes) 
plains,  164-169 
planes,  164-169 

(see  also  Planes) 
zones,  159-161 

(see    also    Zones;      Terminology 
and  classification  of  shores) 
Terraces,  163,  223-224,  405 
backshore  terrace,  163 
continental  terrace,  163 
shoreface  terrace,  163,  259 
storm  terrace,  223 
wave-,  405 

see    also    Ridges,    shore;     Shore 
profile,  development  of;  Ter- 
minology and  classification  of 
shores 
Tidal  currents,  2,  106-122 
deposition  of  debris,  113-115 
hydraulic  currents,  121-122 
movement  of  debris,  115-121 
see  also  Current  action 


582 


INDEX  —  SUBJECTS 


Tidal  delta,  374 

see  also  Deltas 
Tidal  hydraulic  current,  121-122 

see  also  Hydraulic  currents 
Tidal  inlet,  307,  367 

see  also  Inlets 
Tidal  ripples,  498-500 

see  also  Ripple  marks 
Tidal  waves,  2,  41-42 
compound  tidal  wave,  41 
tidal  wavelet,  41-42 
see  also  Waves,  water 
Tide;     see    Tidal     currents;      Tidal 

waves 
Tombolos,  310,  315,  411,  431 
complex  tombolo,  431 
double  tombolo,  315 
the  Swinemiinde,  431 
triple  tombolo,  315 
Y-tombolo,  315 

see  also  Ridges,  shore;  Shoreline, 
development  of 
Translation,  waves  of,  1,  33-38,  408 
complexity,  36-38 
fonn,  33-34 

(see  also  Fonn  of  waves) 
height,  34 

(see  also  Height  of  waves) 
length,  35 

(see  also  Length  of  waves) 
motion,  34-35 

,     (see  also  Motion  of  waves) 
origin,  33 

(see  also  Origin  of  waves) 
velocity,  35-36 

(see    also    Velocity    of    waves; 
Waves,  water) 
Triple  tombolo,  315 

see  also  Tombolos 
Trochoidal  waves,  13 

see  also    Oscillation,    waves    of; 
Waves,  water 
Trimcated  cuspate  foreland,  325 
see  also  Forelands 

Valleuse,  343 

(see  also  Valleys) 


Valley,  drowned,  272 
(see  also  Valleys) 
dune,  404 

(see  also  Valleys) 
hanging,  343  . 
(see  also  Valleys) 
VaUeys,  272,  343,  404 
drownied  valley,  272 
hanging  valley,  343 
valleuse,  343 

see  also  Ridges,  shore;  Shoreline, 
development  of 
V-bar,  322 

see  also  Bars 
Velocity  of  waves,  2,  29-33,  35-36 
waves  of  oscillation,  29-33 

(see  also  Oscillation,  waves  of) 
waves  of  translation,  35-36 

(see  also  Translation,  waves  of) 
Veneer,  163 
Volcanic  shoreline,  188,  263 

Wallen,  the,  422 
Water  line,  159 
Wave  action,  depth  of,  76-83 
Wave  attack,  nature  of,  57-62 
Wave  currents,  90-106 
beach  drifting,  94-103 
hydrauUc  currents,  103-105 
work  of  currents,  105-106 
see  also  Current  Action 
Wave  delta,  306 

see  also  Deltas 
Wave  energy,  55-74 

conditions  affecting  energy,  72- 
74 
measurement  of  energy,  63-65 
Wave  erosion,  161-162 
abrasion  platform,  162 
bench,  162 
cUff,  161 
Wave  dynamometer,  62-63 
Wave-terrace,  405 

see  also  Terraces 
Waves,  capillary,  7 

combined,  25-27,  36-38 
Waves  of  oscillation;   see  Oscillation, 
waves  of 


INDEX  —  SUBJECTS 


583 


Waves  of  Translation;    see  Transla- 
tion, waves  of 
Waves,  orbits  of,  11-12 

storm,  damage  done  by,  65-72 
water,  1-54 

bottom  drag,  16 

boundary  waves,  2,  44-45 

breaker,  16-20 

capillary  waves,  7 

characteristics  of  waves,  1 

combined  waves,  25-27 

combing  waves,  16 

cycloidal  waves,  13 

depth  at  which  waves  break,  1, 

16-20 
earthquake  waves,  2,  38-40 
(see  also  Earthquake  and  Ex- 
plosion waves) 
explosion  waves,  2,  38-41 

(see  also  Earthquake  and  Ex- 
plosion waves) 
form  of  waves,  12-21,  33-34 

(see  also  Form  of  waves) 
ground  swell,  15 
height  of  waves,  21-27,  34 

(see  also  Height  of  waves) 
intersecting  waves,  20-21 
length  of  waves,  27-29,  35 

(see  also  Length  of  waves) 
Uterature  re  waves,  1-7 
motion  of  waves,  8-12,  34-35 
(see  also  Motion  of  waves) 
nature  of  waves,  1 
origin  of  waves,  1,  7-8,  33 

(see  also  Origin  of  waves) 
oscillation  waves,  1,  7-33 

(see  also  Oscillation,  waves  of) 
scope  of  subject,  2-4 
seiches,  42-43 
standing  waves,  42-43 
surf,  16 
swell,  15 
tidal  waves,  2,  41-42 

(see  also  Tidal  waves) 
translation  waves,  1,  33-38 
(see    also    Translation,    waves 
of) 
trochoidal  waves,  13 


Waves,  water,  types  of  waves,  7 

velocity  of  waves,  2,  29-33,  35-36 

(see  also  Velocity  of  waves) 
work  performed  by  waves,  1 
(see  also  Waves,  work  of) 
References,  46-54 

see      also      under      Index  — 
Authors:  Abercromby,  R. 
Airy,   G.   B.;     Antoine,  C. 
Bache,   A.   D.;    Bazin,   H. 
Bertin,    E.;     Bois,    C.    des 
Boussinesq,  J.;   Bremontier 
N.    T.;     Caligny,     A.    de 
Cialdi,    A.;     Cornaglia,    P. 
Cornish,    V.;    Darcy;    Dar- 
win,   G.    H.;     Darwin,    L.; 
Dawson,  W.  B.;  Ekman,  V. 
W.;    Emy,   A.   R.;    Fenne- 
man,  N.  M.;  Fleming,  J.  A.; 
Gaillard,  D.  D.;    Gerstner, 
F.;    Hagen,  G.;    Harris,  R. 
A.;  Haupt,  L.  M.;  Helland- 
Hansen,  B.  and  Nansen,  F. 
Hobbs,  W.H.;  Hunt,  A.  R. 
Kelvin,  Lord;  Kriimmel,  O. 
Lagrange;     Lyman,    C.    S. 
Marsh,  G.  P.;  Moller;  Mot 
tez.  A.;  Newton;  Paris,  A.; 
Rankine,  W.  J.  M.;    Red- 
field;    Reid,   H.   F.;    Ross 
Russell,  J.  S. ;  Saint- Venant 
B.   de;    Sandstrom,   J.   W. 
Schott,   G.;    Scoresby,   W. 
Stevenson,   T.;    Stokes,   G 
G.;   Tanner,  Z.  L.;   Taylor 
F.     B.;      Thompson,     W. 
Thoulet,  J.;    Vinci,   L.   da 
Weber,     E.     H.     and     W. 
Wheeler,    W.    H.;     White, 
W.  H. 
work  of,  55-86 

conditions  affecting  energy,  72-74 
damage  by  storm  waves,  65-72 
depth  of  wave  action,  76-83 
dynamometer,  62-63 
energy  generated  by  waves,  55- 
57 
(see  also  Energy,  wave) 


584 


INDEX  —  SUBJECTS 


Waves,  work  of,  kinetic  energy,  56 
measurement  of  energy,  63-65 
nature  of  wave-attack,  57-62 
potential  energy,  56 
refraction  of  waves,  74r-76 

see  also  Waves,  water 
References,  83-86 

see  also,  under  Index — 
Authors:  Agassiz,  A.;  Airy, 
G.  B.;  Calver,  E.  K.;  Ci- 
aldi,  A.;  Coode,  J.;  Cor- 
nish, v.;  Davis,  W.  M.; 
Delesse,  M.;  Douglas,  J. 
N.;  Ekman,  V.  W.;  Flem- 
ing, J.  A.;  Fol,  H.;  Forbes, 
E.;  GaiUard,  D.  D.;  Gei- 
kie,  A.;  Gilbert,  G.  K.; 
Hagen,  G.;  Harrison,  J.  T.; 
Henwood,  W.  J.;  Hunt,  A. 
E..;  Kinahan,  G.  H.;  Lap- 
parent,  A.  de;  Lyell,  C; 
Matthews,  E.  R.;  Meunier, 
S.;  Murray,  J.;  Nansen,  F.; 
Playfair,  J.;  Royal  Com- 
mission on  Coast  Erosion; 
Shaler,  N.  S.;  Shield,  W.; 
Siau;  Sorby,  H.  C;  Steven- 
son, R.;  Stevenson,  T.; 
Thoulet,  J.;  Weber,  E.  H. 
and  W.;  Wheeler,  W.  H. 


Wind  currents,  123-128 
hydraulic  current,  124-125 
seasonal  current,  126-128 
temporary,  125-126 
see  also  Current  action 
Wind  dunes;  see  Dimes,  shore 
Wind  hydraulic  current,  124-125 

see  also  Hydraulic  currents 
Winged  headland,  303 
Winter  full,  411 

see  also  Fulls 
Work,     of     currents     (see     Current 
action) 
of  waves  {see  Waves,  work  of) 

Young  stage;   see  Stages  in  develof)- 

ment  of  shores 
Y-tombolo,  315 

see  also  Tombolos 

Zone  of  equilibrium,  300 
Zones,  159-161 
coast,  159-160 
offshore,  161 
shore,  159-160 
backshore,  161 
foreshore,  161 
shoreface,  161 
see  also  Terminology  and  classi- 
fication of  shores 


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